Mr. Justice Clark

delivered the opinion of the Court.

This appeal challenges the constitutional validity of two sections of an ordinance of the City of Madison, Wisconsin, regulating the sale of milk and milk products within the municipality’s jurisdiction. One section in issue makes it unlawful to sell any milk as pasteurized unless it has been processed and bottled at an approved pasteurization plant within a radius of five miles from the central square of Madison.1 Another section, which prohibits the sale of milk, or the importation, receipt or storage of milk for sale, in Madison unless from a source of supply possessing a permit issued after inspection by Madison officials, is attacked insofar as it expressly relieves municipal authorities from any duty to inspect farms *351located beyond twenty-five miles from the center of the city.2

Appellant is an Illinois corporation engaged in distributing milk and milk products in Illinois and Wisconsin. It contended below, as it does here, that both the five-mile limit on pasteurization plants and the twenty-five-mile limit on sources of milk violate the Commerce Clause and the Fourteenth Amendment to the Federal Constitution. The Supreme Court of Wisconsin upheld the five-mile limit on pasteurization.3 As to the twenty-five-mile limitation the court ordered the complaint dismissed for want of a justiciable controversy. 257 Wis. 308, 43 N. W. 2d 480 (1950). This appeal, contesting both rulings, invokes the jurisdiction of this Court under 28 U. S. C. § 1257 (2).

The City of Madison is the county seat of Dane County. Within the county are some 5,600 dairy farms with total *352raw milk production in excess of 600,000,000 pounds annually and more than ten times the requirements of Madison. Aside from the milk supplied to Madison, fluid milk produced in the county moves in large quantities to Chicago and more distant consuming areas, and the remainder is used in making cheese, butter and other products. At the time of trial the Madison milkshed was not of “Grade A” quality by the standards recommended by the United States Public Health Service, and no milk labeled “Grade A” was distributed in Madison.

The area defined by the ordinance with respect to milk sources encompasses practically all of Dane County and includes some 500 farms which supply milk for Madison. Within the five-mile area for pasteurization are plants of five processors, only three of which are engaged in the general wholesale and retail trade in Madison. Inspection of these farms and plants is scheduled once every thirty days and is performed by two municipal inspectors, one of whom is full-time. The courts below found that the ordinance in question promotes convenient, economical and efficient plant inspection.

Appellant purchases and gathers milk from approximately 950 farms in northern Illinois and southern Wisconsin, none being within twenty-five miles of Madison. Its pasteurization plants are located at Chemung and Huntley, Illinois, about 65 and 85 miles respectively from Madison. Appellant was denied a license to sell its products within Madison solely because its pasteurization plants were more than five miles away.

It is conceded that the milk which appellant seeks to sell in Madison is supplied from farms and processed in plants licensed and inspected by public health authorities of Chicago, and is labeled “Grade A” under the Chicago ordinance which adopts the rating standards recommended by the United States Public Health Serv*353ice. Both the Chicago and Madison ordinances, though not the sections of the latter here in issue, are largely patterned after the Model Milk Ordinance of the Public Health Service. However, Madison contends and we assume that in some particulars its ordinance is more rigorous than that of Chicago.

Upon these facts we find it necessary to determine only the issue raised under the Commerce Clause, for we agree with appellant that the ordinance imposes an undue burden on interstate commerce.

This is not an instance in which an enactment falls because of federal legislation which, as a proper exercise of paramount national power over commerce, excludes measures which might otherwise be within the police power of the states. See Currin v. Wallace, 306 U. S. 1, 12-13 (1939). There is no pertinent national regulation by the Congress, and statutes enacted for the District of Columbia indicate that Congress has recognized- the appropriateness of local regulation of the sale of fluid milk. D. C. Code, 1940, §§ 33-301 et seq. It is not contended, however, that Congress has authorized the regulation before us.

Nor can there be objection to the avowed purpose of this enactment. We assume that difficulties in sanitary regulation of milk and milk products originating in remote areas may present a situation in which “upon a consideration of all the relévant facts and circumstances it appears that the matter is one which may appropriately be regulated in the interest of the safety, health and well-being of local communities . . . Parker v. Brown, 317 U. S. 341, 362-363 (1943); see H. P. Hood & Sons v. Du Mond, 336 U. S. 525, 531-532 (1949). We also assume that since Congress has not spoken to the contrary, the subject matter of the ordinance lies within the sphere of state regulation even though interstate com*354merce may be affected. Milk Control Board v. Eisenberg Farm Products, 306 U. S. 346 (1939); see Baldwin v. Seelig, Inc., 294 U. S. 511, 524 (1935).

But this regulation, like the provision invalidated in Baldwin v. Seelig, Inc., supra, in practical effect excludes from distribution in Madison wholesome milk produced and pasteurized in Illinois. “The importer . . . may keep his milk or drink it, but sell it he may not.” Id., at 521. In thus erecting an economic barrier protecting a major local industry against competition from without the State, Madison plainly discriminates against interstate commerce.4 This it cannot do, even in the exercise of its unquestioned power to protect the health and safety of its people, if reasonable nondiscriminatory alternatives, adequate to conserve legitimate local interests, are available. Cf. Baldwin v. Seelig, Inc., supra, at 524; Minnesota v. Barber, 136 U. S. 313, 328 (1890). A different view, that the ordinance is valid simply because it professes to be a health measure, would mean that the Commerce Clause of itself imposes no limitations on state action other than those laid down by the Due Process Clause, save for the rare instance where a state artlessly discloses an avowed purpose to discriminate against interstate goods. Cf. H. P. Hood & Sons v. Du Mond, supra. Our issue then is whether the discrimination inherent, in the Madison ordinance can be justified in view of the character of the local interests and the available methods of protecting them. Cf. Union Brokerage Co. v. Jensen, 322 U. S. 202, 211 (1944).

It appears that reasonable and adequate alternatives are available. If the City of Madison prefers to rely upon its own officials for inspection of distant milk *355sources, such inspection is readily open to it without hardship for it could charge the actual and reasonable cost of such inspection to the importing producers and processors. Cf. Sprout v. City of South Bend, 277 U. S. 163, 169 (1928); see Miller v. Williams, 12 F. Supp. 236, 242, 244 (D. Md. 1935). Moreover, appellee Health Commissioner of Madison testified that as proponent of the local milk ordinance he had submitted the provisions here in controversy and an alternative proposal based on § 11 of the Model Milk Ordinance recommended by the United States Public Health Service. The model provision imposes no geographical limitation on location of milk sources and processing plants but excludes from the municipality milk not produced and pasteurized conformably to standards as high as those enforced by the receiving city.5 In implementing such an ordinance, the importing city obtains milk ratings based on uniform standards and established by health authorities in the jurisdiction where production and processing occur. The receiving city may *356determine the extent of enforcement of sanitary standards in the exporting area by verifying the accuracy of safety ratings of specific plants or of the milkshed in the distant jurisdiction through the United States Public Health Service, which routinely and on request spot checks the local ratings. The Commissioner testified that Madison consumers “would be safeguarded adequately” under either proposal and that he had expressed no preference. The milk sanitarian of the Wisconsin State Board of Health testified that the State Health Department recommends the adoption of a provision based on the Model Ordinance. Both officials agreed that a local health officer would be justified in relying upon the evaluation by the Public Health Service of enforcement conditions in remote producing areas.

To permit Madison to adopt a regulation not essential for the protection of local health interests and placing a discriminatory burden on interstate commerce would invite a multiplication of preferential trade areas destructive of the very purpose of the Commerce Clause. Under the circumstances here presented, the regulation must yield to the principle that “one state in its dealings with another may not place itself in a position of economic isolation.” Baldwin v. Seelig, Inc., supra, at 527.

For these reasons we conclude that the judgment below sustaining the five-mile provision as to pasteurization must be reversed.

The Supreme Court of Wisconsin thought it unnecessary to pass upon the validity of the twenty-five-mile limitation, apparently in part for the reason that this issue was made academic by its decision upholding the five-mile section. In view of our conclusion as to the latter provision, a determination of appellant's contention as to the other section is now necessary. As to this *357issue, therefore, we vacate the judgment below and remand for further proceedings not inconsistent with the principles announced in this opinion.

It is so ordered.

Mr. Justice Black,

with whom Mr. Justice Douglas and Mr. Justice Minton concur, dissenting.

Today’s holding invalidates § 7.21 of the Madison, Wisconsin, ordinance on the following reasoning: (1) the section excludes wholesome milk coming from Illinois; (2) this imposes a discriminatory burden on interstate commerce; (3) such a burden cannot be imposed where, as here, there are reasonable, nondiscriminatory and adequate alternatives available. I disagree with the Court’s premises, reasoning, and judgment.

(1) This ordinance does not exclude wholesome milk coming from Illinois or anywhere else. It does require that all milk sold in Madison must be pasteurized within five miles of the center of the city. But there was no finding in the state courts, nor evidence to justify a finding there or here, that appellant, Dean Milk Company, is unable to have its milk pasteurized within the defined geographical area. As a practical matter, so far as the record shows, Dean can easily comply with the ordinance whenever it wants to. Therefore, Dean’s personal preference to pasteurize in Illinois, not the ordinance, keeps Dean’s milk out of Madison.

(2) Characterization of § 7.21 as a “discriminatory burden” on interstate commerce is merely a statement of the Court’s result, which I think incorrect. The section does prohibit the sale of milk in Madison by interstate and intrastate producers who prefer to pasteurize over five miles distant from the city. But both state courts below found that § 7.21 represents a good-faith attempt to safeguard public health by making adequate sanitation *358inspections possible. While we are not bound by these findings, I do not understand the Court to overturn them. Therefore, the fact that § 7.21, like all health regulations, imposes some burden on trade, does not mean that it “discriminates” against interstate commerce.

(3) This health regulation should not be invalidated merely because the Court believes that alternative milk-inspection methods might insure the cleanliness and healthfulness of Dean’s Illinois milk. I find it difficult to explain why the Court uses the “reasonable alternative” concept to protect trade when today it refuses to apply the same principle to protect freedom of speech. Feiner v. New York, 340 U. S. 315. For while the “reasonable alternative” concept has been invoked to protect First Amendment rights, e. g., Schneider v. State, 308 U. S. 147, 162, it has not heretofore been considered an appropriate weapon for striking down local health laws. Since the days of Chief Justice Marshall, federal courts have left states and municipalities free to pass bona fide health regulations subject only “to the paramount authority of Congress if it decides to assume control . . . .” The Minnesota Rate Cases, 230 U. S. 352, 406; Gibbons v. Ogden, 9 Wheat. 1, 203, 204; Mintz v. Baldwin, 289 U. S. 346, 349-350; and see Baldwin v. Seelig, 294 U. S. 511, 524. This established judicial policy of refusing to invalidate genuine local health laws under the Commerce Clause has been approvingly noted even in our recent opinions measuring state regulation by stringent standards. See, e. g., Hood & Sons v. Du Mond, 336 U. S. 525, 531-532. No case is cited, and I have found none, in which a bona fide health law was struck down on the ground that some other method of safeguarding health would be as good as, or better than, the one the Court was called on to review. In my view, to use this ground now elevates the right to traffic in commerce for profit above *359the power of the people to guard the purity of their daily diet of milk.

If, however, the principle announced today is to be followed, the Court should not strike down local health regulations unless satisfied beyond a reasonable doubt that the substitutes it proposes would not lower health standards. I do not think that the Court can so satisfy itself on the basis of its judicial knowledge. And the evidence in the record leads me to the conclusion that the substitute health measures suggested by the Court -do not insure milk as safe as the Madison ordinance requires.

One of the Court’s proposals is that Madison require milk processors to pay reasonable inspection fees at the milk supply “sources.” Experience shows, however, that the fee method gives rise to prolonged litigation over the calculation and collection of the charges. E. g., Sprout v. South Bend, 277 U. S. 163; Capitol Greyhound Lines v. Brice, 339 U. S. 542. To throw local milk regulation into such a quagmire of uncertainty jeopardizes the admirable milk-inspection systems in force in many municipalities. Moreover, nothing in the record before us indicates that the fee system might not be as costly to Dean as having its milk pasteurized in Madison. Surely the Court is not resolving this question by drawing on its “judicial knowledge” to supply information as to comparative costs, convenience, or effectiveness.

The Court’s second proposal is that Madison adopt § 11 of the “Model Milk Ordinance.” The state courts made no findings as to the relative merits of this inspection ordinance and the one chosen by Madison. The evidence indicates to me that enforcement of the Madison law would assure a more healthful quality of milk than that which is entitled to use the label of “Grade A” under the Model Ordinance. Indeed, the United States Board of Public Health, which drafted the Model Ordinance, sug*360gests that the provisions are “minimum” standards only. The Model Ordinance does not provide for continuous investigation of all pasteurization plants as does § 7.21 of the Madison ordinance. Under § 11, moreover, Madison would be required to depend on the Chicago inspection system since Dean’s plants, and the farms supplying them with raw milk, are located in the Chicago milkshed. But there is direct and positive evidence in the record that milk produced under Chicago standards did not meet the Madison requirements.

Furthermore, the Model Ordinance would force the Madison health authorities to rely on “spot checks” by the United States Public Health Service to determine whether Chicago enforced its milk regulations. The evidence shows that these “spot checks” are based on random inspection of farms and pasteurization plants: the United States Public Health Service rates the ten thousand or more dairy farms in the Chicago milkshed by a sampling of no more than two hundred farms. The same sampling technique is employed to inspect pasteurization plants. There was evidence that neither the farms supplying Dean with milk nor Dean’s pasteurization plants were necessarily inspected in the last “spot check” of the Chicago milkshed made two years before the present case was tried.

From what this record shows, and from what it fails to show, I do not think that either of the alternatives suggested by the Court would assure the people of Madison as pure a supply of milk as they receive under their own ordinance. On this record I would uphold the Madison law. At the very least, however, I would not invalidate it without giving the parties a chance to present evidence and get findings on the ultimate issues the Court thinks crucial — namely, the relative merits of the Madison ordinance and the alternatives suggested by the Court today.

Mr. Justice Blackmun

delivered the opinion of the Court.

The issue in this case is whether, consistent with the Commerce Clause, U. S. Const., Art. I, § 8, cl. 3, the State of South Dakota, in a time of shortage, may confine the sale of the cement it produces solely to its residents.

I

In 1919, South Dakota undertook plans to build a cement plant. The project, a product of the State’s then prevailing Progressive political movement, was initiated in response to recent regional cement shortages that “interfered with and delayed both public and private enterprises,” and that were “threatening the people of this state.” Eakin v. South Dakota State Cement Comm’n, 44 S. D. 268, 272, 183 N. W. 651, 652 *431(1921).1 In 1920, the South Dakota Cement Commission anticipated “[t]hat there would be a ready market for the. entire output of the plant within the state.” Report of State *432Cement Commission 9 (1920). The plant, however, located at Rapid City, soon produced more cement than South Da-kotans could use. Over the years, buyers in no less than nine nearby States purchased cement from the State’s plant. App. 26. Between 1970 and 1977, some 40% of the plant’s output went outside the State.

The plant’s list of out-of-state cement buyers included petitioner Reeves, Inc. Reeves is a ready-mix concrete2 distributor organized under Wyoming law and with facilities in Buffalo, Gillette, and Sheridan, Wyo. Id,., at 15. From the beginning of its operations in 1958, and until 1978, Reeves purchased about 95% of its cement from the South Dakota plant. Id., at 15 and 22. In 1977, its purchases were $1,172,000. Id., at 17. In turn, Reeves has supplied three northwestern Wyoming counties with more than half their ready-mix concrete needs. Id., at 15. For 20 years the relationship between Reeves and the South Dakota cement plant was amicable, uninterrupted, and mutually profitable.

As the 1978 construction season approached, difficulties at the plant slowed production. Meanwhile, a booming construction industry spurred demand for cement both regionally and nationally. Id., at 13. The plant found itself unable to meet all orders. Faced with the same type of “serious cement shortage” that inspired the plant’s construction, the Commission “reaffirmed its policy of supplying all South Dakota customers first and to honor all contract commit*433ments, with the remaining volume allocated on a first come, first served basis.” Ibid 3

Reeves, which had no pre-existing long-term supply contract, was hit hard and quickly by this development. On June 30, 1978, the plant informed Reeves that it could not continue to fill Reeves’ orders, and on July 5, it turned away a Reeves truck. Id., at 17-18. Unable to find another supplier, id., at 21, Reeves was forced to cut production by 76% in mid-July. . Id., at 20.

On July 19, Reeves brought this suit against the Commission, challenging the plant’s policy of preferring South Dakota buyers, and seeking injunctive relief. Id., at 3-10. After conducting a hearing and receiving briefs and affidavits, the District Court found no substantial issue of material fact and permanently enjoined the Commission’s practice. The court reasoned that South Dakota’s “hoarding” was inimical to the national free market envisioned by the Commerce Clause. Id., at 27-30.

The United States Court of Appeals for the Eighth Circuit reversed. Reeves, Inc. v. Kelley, 586 F. 2d 1230, 1232 (1978). It concluded that the State had “simply acted in a proprietary capacity,” as permitted by Hughes v. Alexandria Scrap Corp., 426 U. S. 794 (1976). Petitioner sought certiorari. This Court granted the petition, vacated the judgment, and remanded the case for further consideration in light of Hughes v. Oklahoma, 441 U. S. 322 (1979). Reeves, Inc. v. Kelley, 441 U. S. 939 (1979). On remand, the Court of Appeals distinguished that case.4 Again relying on Alexandria *434 Scrap, the court abided by its previous holding. Reeves, Inc. v. Kelley, 603 F. 2d 736 (1979). We granted Reeves’ petition for certiorari to consider once again the impact of the Commerce Clause on state proprietary activity. 444 U. S. 1031 (1980).5

II

A

Alexandria Scrap concerned a Maryland program designed to remove abandoned automobiles from the State’s roadways and junkyards. To encourage recycling, a “bounty” was offered for every Maryland-titled junk car converted into scrap. Processors located both in and outside Maryland were eligible to collect these subsidies. The legislation, as initially enacted in 1969, required a processor seeking a bounty to present documentation evidencing ownership of the wrecked car. This requirement however, did not apply to “hulks,” inoperable automobiles over eight years old. In 1974, the statute was amended to extend documentation requirements to hulks, which comprised a large majority of the junk cars being processed. Departing from prior practice, the new law imposed more exacting documentation requirements on out-of-state than in-state processors. By making it less remunerative for suppliers to transfer vehicles outside Maryland, the *435reform triggered a “precipitate decline in the number of bounty-eligible hulks supplied to appellee’s [Virginia] plant from Maryland sources.” 426 U. S., at 801. Indeed, “[t]he practical effect was substantially the same as if Maryland had withdrawn altogether the availability of bounties on hulks delivered by unlicensed suppliers to licensed non-Maryland processors.” Id., at 803, n. 13; see id., at 819 (dissénting opinion).

Invoking the Commerce Clause, a three-judge District Court struck down the legislation. 391 F. Supp. 46 (Md. 1975). It observed that the amendment imposed “substantial burdens upon the free flow of interstate commerce,” id., at 62, and reasoned that the discriminatory program was not the least disruptive means of achieving the State’s articulated objective. Id., at 63. See generally Pike v. Bruce Church, Inc., 397 U. S. 137, 142 (1970).6

This Court reversed. It recognized the persuasiveness of the lower court’s analysis if the inherent restrictions of the Commerce Clause were deemed applicable. In the Court’s view, however, Alexandria Scrap did not involve “the kind of action with which the Commerce Clause is concerned.” 426 U. S., at 805. Unlike prior cases voiding state laws inhibiting interstate trade, “Maryland has not sought to prohibit the flow of hulks, or to regulate the conditions under which it may occur. Instead, it has entered into the market itself to bid up their price,” id., at 806, “as a purchaser, in effect, of a potential article of interstate commerce,” and has restricted “its trade to its own citizens or businesses within the State.” Id., at 808.7

*436Having characterized Maryland as a market participant, rather than as a market regulator, the Court found no reason to “believe the Commerce Clause was intended to require independent justification for [the State’s] action.” Id., at 809. The Court couched its holding in unmistakably broad terms. “Nothing in the purposes animating the Commerce Clause prohibits a State, in the absence of congressional action, from participating in the market and exercising the right to favor its own citizens over others.” Id., at 810 (footnote omitted).8

B

The basic distinction drawn in Alexandria Scrap between States as market participants and States as market regulators makes good sense and sound law. As that case explains, the *437Commerce Clause responds principally to state taxes and regulatory measures impeding free private trade in the national marketplace. Id., at 807-808, citing. H. P. Hood & Sons v. Du Mond, 336 U. S. 525, 539 (1949) (referring to “home embargoes,” “customs duties,” and “regulations” excluding imports). There is no indication of a constitutional plan to limit the ability of the States themselves to operate freely in the free market. See L. Tribe, American Constitutional Law 336 (1978) (“the commerce clause was directed, as an historical matter, only at regulatory and taxing actions taken by states in their sovereign capacity”). The precedents comport with this distinction.9

*438Restraint in this area is also counseled by considerations of state sovereignty,10 the role of each State “ 'as guardian and trustee for its people,’ ” Heim v. McCall, 239 U. S. 175, 191 (1915), quoting Atkin v. Kansas, 191 U. S. 207, 222-223 (1903),11 and "the long recognized right of trader or manu*439facturer, engaged in an entirely private business, freely to exercise his own independent discretion as to parties with whom he will deal.” United States v. Colgate & Co., 250 U. S. 300, 307 (1919).12 Moreover, state proprietary activities may be, and often are, burdened with the same restrictions imposed on private market participants.13 Evenhandedness suggests that, when acting as proprietors, States should similarly share existing freedoms from federal constraints, including the inherent limits of the Commerce Clause. See State ex rel. Collins v. Senatobia Blank Book & Stationery Co., 115 Miss. 254, 260, 76 So. 258, 260 (1917); Tribune Printing & Binding Co. v. Barnes, 7 N. D. 591, 597, 75 N. W. 904, 906 (1898). Finally, as this case illustrates, the competing considerations in cases involving state proprietary action often will be subtle, complex, politically charged, and difficult to assess under traditional Commerce Clause analysis. Given these factors, Alexandria Scrap wisely recognizes that, as a rule, the adjustment of interests in this context is a task better suited for Congress than this Court.

*440III

South Dakota, as a seller of cement, unquestionably fits the “market participant” label more comfortably than a State acting to subsidize local scrap processors. Thus, the general rule of Alexandria Scrap plainly applies here.14 Petitioner argues, however, that the exemption for marketplace participation necessarily admits of exceptions. While conceding that possibility, we perceive in this case no sufficient reason to depart from the general rule.

A

In finding a Commerce Clause violation, the District Court emphasized “that the Commission . . . made an election to become part of the interstate commerce system.” App. 28. The gist of this reasoning, repeated by petitioner here, is that one good turn deserves another. Having long exploited the interstate market, South Dakota should not be permitted to withdraw from it when a shortage arises. This argument is not persuasive. It is somewhat self-serving to say that South Dakota has “exploited” the interstate market. An equally fair characterization is that neighboring States long have benefited from South Dakota's foresight and industry. Viewed in this light, it is not surprising that Alexandria Scrap rejected an argument that the 1974 Maryland legislation challenged there was invalid because cars abandoned in Maryland had been processed in neighboring States for five years. As in Alexandria Scrap, we must conclude that “this chronology does not distinguish the case, for Commerce Clause purposes, *441from one in which a State offered [cement] only to domestic [buyers] from the start.” 426 IT. S., at 809.15

Our rejection of petitioner’s market-exploitation theory fundamentally refocuses analysis. It means that to reverse we would have to void a South Dakota “residents only” policy even if it had been enforced from the plant’s very first days. Such a holding, however, would interfere significantly with a State’s ability to structure relations exclusively with its own citizens. It would also threaten the future fashioning of effective and creative programs for solving local problems and distributing government largesse. See n. 1, supra. A healthy regard for federalism and good government renders us reluctant to risk these results.

“To stay experimentation in things social and economic is a grave responsibility. Denial of the right to experiment may be fraught with serious consequences to the Nation. It is one of the happy incidents of the federal system that a single courageous State may, if its citizens choose, serve as a laboratory; and try novel social and economic experiments without risk to the rest of the country.” New State Ice Co. v. Liebmann, 285 U. S. 262, 311 (1932) (Brandéis, J., dissenting).

*442B

Undaunted by these considerations, petitioner advances four more arguments for reversal:

First, petitioner protests that South Dakota’s preference for its residents responds solely to the “non-governmental ob-jectiv[e]” of protectionism. Brief for Petitioner 25. Therefore, petitioner argues, the policy is per se invalid. See Philadelphia v. New Jersey, 437 U. S. 617, 624 (1978).

We find the label “protectionism” of little help in this context. The State’s refusal to sell to buyers other than South Dakotans is “protectionist” only in the sense that it limits benefits generated by a state program to those who fund the state treasury and whom the State was created to serve. Petitioner’s argument apparently also would characterize as “protectionist” rules restricting to state residents the enjoyment of state educational institutions, energy generated by a state-run plant, police and fire protection, and agricultural improvement and business development programs. Such policies, while perhaps “protectionist” in a loose sense, reflect the essential and patently unobjectionable purpose of state government — to serve the citizens of the State.16

*443Second, petitioner echoes the District Court’s warning:

“If a state in this union, were allowed to hoard its commodities or resources for the use of their own residents only, a drastic situation might evolve. For example, Pennsylvania or Wyoming might keep their coal, the northwest its timber, and the mining states their minerals. The result being that embargo may be retaliated by embargo and commerce would be halted at state lines.” App. 29.

See, e. g., Baldwin v. Montana Fish & Game Comm’n, 436 U. S. 371, 385-386 (1978). This argument, although rooted in the core purpose of the Commerce Clause, does not fit the present facts. Cement is not a natural resource, like coal, timber, wild game, or minerals. Cf. Hughes v. Oklahoma, 441 U. S. 322 (1979) (minnows); Philadelphia v. New Jersey, supra (landfill sites); Pennsylvania v. West Virginia, 262 U. S. 553 (1923) (natural gas); West v. Kansas Natural Gas *444 Co., 221 U. S. 229 (1911) (same); Note, 32 Rutgers L. Rev. 741 (1979). It is the end product of a complex process whereby a costly physical plant and human labor act on raw materials. South Dakota has not sought to limit access to the State's limestone or other materials used to make cement. Nor has it restricted the ability of private firms or sister States to set up plants within its borders. Tr. of Oral Arg. 4. Moreover, petitioner has not suggested that South Dakota possesses unique access to the materials needed to produce cement.17 Whatever limits might exist on a State’s ability to invoke the Alexandria Scrap exemption to hoard resources which by happenstance are found there, those limits do not apply here.

Third, it is suggested that the South Dakota program is infirm because it places South Dakota suppliers of ready-mix concrete at a competitive advantage in the out-of-state market; Wyoming suppliers, such as petitioner, have little chance against South Dakota suppliers who can purchase cement from the State’s plant and freely sell beyond South Dakota’s borders.

The force of this argument is seriously diminished, if not eliminated, by several considerations. The argument neces*445sarily implies that the South Dakota scheme would be unobjectionable if sales in other States were totally barred. It therefore proves too much, for it would tolerate even a greater measure of protectionism and stifling of interstate commerce than the challenged system allows. See K. S. B. Technical Sales Corp. v. North Jersey Dist. Water Supply Comm’n, 75 N. J. 272, 298, 381 A. 2d 774, 787 (1977) (“It would be odd indeed to find that when a state becomes less parochial . . . its purpose becomes suspect under the Commerce Clause”). Cf. Pike v. Bruce Church, Inc., 397 U. S., at 142 (“And the extent of the burden that will be tolerated will of course depend ... on whether [the state interest] could be promoted as well with a lesser impact on interstate activities”). Nor is it to be forgotten that Alexandria Scrap approved a state program that “not only . . . effectively protect [ed] scrap processors with existing plants in Maryland from the pressures of competitors with nearby out-of-state plants, but [that] implicitly offer [ed] to extend similar protection to any competitor . . . willing to erect a scrap processing facility within Maryland’s boundaries.” 391 F. Supp., at 63. Finally, the competitive plight of out-of-state ready-mix suppliers cannot be laid solely at the feet of South Dakota. It is attributable as well to their own States’ not providing or attracting alternative sources of supply and to the suppliers’ own failure to guard against shortages by executing long-term supply contracts with the South Dakota plant.

In its last argument, petitioner urges that, had South Dakota not acted, free market forces would have generated an appropriate level of supply at free market prices for all buyers in the region. Having replaced free market forces, South Dakota should be forced to replicate how the free market would have operated under prevailing conditions.

This argument appears to us to be simplistic and speculative. The very reason South Dakota built its plant was because the free market had failed adequately to supply the *446region with cement. See n. 1, supra. There is no indication, and no way to know, that private industry would have moved into petitioner’s market area, and would have ensured a supply of cement to petitioner either prior to or during the 1978 construction season. Indeed, it is quite possible that petitioner would never have existed — far less operated successfully for 20 years — had it not been for South Dakota cement.18

C

We conclude, then, that the arguments for invalidating South Dakota’s resident-preference program are weak at best. Whatever residual force inheres in them is more than offset by countervailing considerations of policy and fairness. Reversal would discourage similar state projects, even though this project demonstrably has served the needs of state residents and has helped the entire region for more than a half century. Reversal also would rob South Dakota of the intended benefit of its foresight, risk, and industry.19 Under *447these circumstances, there is no reason to depart from the general rule of Alexandria Scrap.

The judgment of the United States Court of Appeals is affirmed.

It is so ordered.

Mr. Justice Powell,

with whom Mr. Justice Brennan, Mr. Justice White, and Mr. Justice Stevens join, dissenting.

The South Dakota Cement Commission has ordered that in times of shortage the state cement plant must turn away out-of-state customers until all orders from South Dakotans are filled. This policy represents precisely the kind of economic protectionism that the Commerce Clause was intended to prevent.1 The Court, however, finds no violation of the Commerce Clause, solely because the State produces the cement. I agree with the Court that the State of South Dakota may provide cement for its public needs without violating the Commerce Clause. But I cannot agree that South Dakota may withhold its cement from interstate commerce in order to benefit private citizens and businesses within the State.

I

The need to ensure unrestricted trade among the States created a major impetus for the drafting of the Constitution. “The power over commerce . . . was one of the primary objects for which the people of America adopted their government. . . .” Gibbons v. Ogden, 9 Wheat. 1, 190 (1824). Indeed, the Constitutional Convention was called after an earlier convention on trade and commercial problems proved inconclusive. C. Beard, An Economic Interpretation of the *448Constitution 61-63 (1935); S. Bloom, History of the Formation of the Union Under the Constitution 14-15 (1940). In the subsequent debate over ratification, Alexander Hamilton emphasized the importance of unrestricted interstate commerce:

“An unrestrained intercourse between the States themselves will advance the trade of each, by an interchange of their respective productions. . . . Commercial enterprise will have much greater scope, from the diversity in the productions of different States. When the staple of one fails ... it can call to its aid the staple of another.” The Federalist, No. 11, p. 71 (J. Cooke ed., 1961) (A. Hamilton); see id., No. 42, p. 283 (J. Madison).

The Commerce Clause has proved an effective weapon against protectionism. The Court has used it to strike down limitations on access to local goods, be they animal, Hughes v. Oklahoma, 441 U. S. 322 (1979) (minnows); vegetable, Pike v. Bruce Church, Inc., 397 U. S. 137 (1970) (cantaloupes); or mineral, Pennsylvania v. West Virginia, 262 U. S. 553 (1923) (natural gas). Only this Term, the Court held unconstitutional a Florida statute designed to exclude out-of-state investment advisers. Lewis v. BT Investment Managers, Inc., ante, p. 27. As we observed in Hughes v. Alexandria Scrap Corp., 426 U. S. 794, 803 (1976), “this Nation is a common market in which state lines cannot be made barriers to the free flow of both raw materials and finished goods in response to the economic laws of supply and demand.”

This case presents a novel constitutional question. The Commerce Clause would bar legislation imposing on private parties the type of restraint on commerce adopted by South Dakota. See Pennsylvania v. West Virginia, supra; cf. Great A&P Tea Co. v. Cottrell, 424 U. S. 366 (1976); Foster-Fountain Packing Co. v. Haydel, 278 U. S. 1 (1928).2 Conversely, *449a private business constitutionally could adopt a marketing policy that excluded customers who come from another State. This case falls between those polar situations. The State, through its Commission, engages in a commercial enterprise and restricts its own interstate distribution. The question is whether the Commission’s policy should be treated like state regulation of private parties or like the marketing policy of a private business.

The application of the Commerce Clause to this case should turn on the nature of the governmental activity involved. If a public enterprise undertakes an “integral operatio[n] in areas of traditional governmental functions,” National League of Cities v. Usery, 426 U. S. 833, 852 (1976), the Commerce Clause is not directly relevant. If, however, the State enters *450the private market and operates a commercial enterprise for the advantage of its private citizens, it may not evade the constitutional policy against economic Balkanization.

This distinction derives from the power of governments to supply their own needs, see Perkins v. Lukens Steel Co., 310 U. S. 113, 127 (1940); Atkin v. Kansas, 191 U. S. 207 (1903), and from the purpose of the Commerce Clause itself, which is designed to protect “the natural functioning of the interstate market,” Hughes v. Alexandria Scrap Corp., supra, at 806. In procuring goods and services for the operation of government, a State may act without regard to the private marketplace and remove itself from the reach of the Commerce Clause. See American Yearbook Co. v. Askew, 339 F. Supp. 719 (MD Fla.), summarily aff’d, 409 U. S. 904 (1972). But when a State itself becomes a participant in the private market for other purposes, the Constitution forbids actions that would impede the flow of interstate commerce. These categories recognize no more than the “constitutional line between the State as government and the State as trader.” New York v. United States, 326 U. S. 572, 579 (1946); see United States v. California, 297 U. S. 175 (1936); Ohio v. Helvering, 292 U. S. 360 (1934); South Carolina v. United States, 199 U. S. 437 (1905).

The Court holds that South Dakota, like a private business, should not be governed by the Commerce Clause when it enters the private market. But precisely because South Dakota is a State, it cannot be presumed to behave like an enterprise “ 'engaged in an entirely private business.’ ” See ante, at 439, quoting United States v. Colgate & Co., 250 U. S. 300, 307 (1919). A State frequently will respond to market conditions on the basis of political rather than economic concerns. To use the Court’s terms, a State may attempt to act as a “market regulator” rather than a “market participant.” See ante, at 436. In that situation, it is a pretense to equate the State with a private economic actor. State action burdening interstate trade is no less state action because it is *451accomplished by a public agency authorized to participate in the private market.

II

The threshold issue is whether South Dakota has undertaken integral government operations in an area of traditional governmental functions, or whether it has participated in the marketplace as a private firm. If the latter characterization applies, we also must determine whether the State Commission’s marketing policy burdens the flow of interstate trade. This analysis highlights the differences between the state action here and that before the Court in Hughes v. Alexandria Scrap Corp.

A

In Alexandria Scrap, a Virginia scrap processor challenged a Maryland program to pay bounties for every junk car registered in Maryland that was converted into scrap. The program imposed more onerous documentation standards on non-Maryland processors, thereby diverting Maryland “hulks” to in-state processors. The Virginia plaintiff argued that this diversion burdened interstate commerce.

As the Court today notes, Alexandria Scrap determined that Maryland’s bounty program constituted direct state participation in the market for automobile hulks. Ante, at 435. But the critical question — the second step in the opinion’s analysis — was whether the bounty program constituted an impermissible burden on interstate commerce. Recognizing that the case did not fit neatly into conventional Commerce Clause theory, 426 U. S., at 807, we found no burden on commerce.

The Court first observed:

“Maryland has not sought to prohibit the flow of hulks, or to regulate the conditions under which it may occur. Instead, it has entered into the market itself to bid up their price. There has been an impact upon the interstate flow of hulks only because . . . Maryland effectively *452has made it more lucrative for unlicensed suppliers to dispose of their hulks in Maryland. . . Id., at 806.

We further stated “that the novelty of this case is not its presentation of a new form of ‘burden’ upon commerce, but that appellee should characterize Maryland’s action as a burden which the Commerce Clause was intended to make suspect.” Id., at 807. The opinion then emphasized that “no trade barrier of the type forbidden by the Commerce Clause, and involved in previous cases, impedes th[e] movement [of hulks] out of State.” Id., at 809-810. Rather, the hulks “remain within Maryland in response to market forces, including that exerted by money from the State.” Id., at 810. The Court concluded that the subsidies provided under the Maryland program erected no barriers to trade. Consequently, the Commerce Clause did not forbid the Maryland program.

B

Unlike the market subsidies at issue in Alexandria Scrap, the marketing policy of the South Dakota Cement Commission has cut off interstate trade.3 The State can raise such a bar when it enters the market to supply its own needs. In order to ensure an adequate supply of cement for public uses, the State can withhold from interstate commerce the cement needed for public projects. Cf. National League of Cities v. Usery, supra.

The State, however, has no parallel justification for favoring private, in-state customers over out-of-state customers.4 *453In response to political concerns that likely would be inconsequential to a private cement producer, South Dakota has shut off its cement sales to customers beyond its borders. That discrimination constitutes a direct barrier to trade “of the type forbidden by the Commerce Clause, and involved in previous cases....” Alexandria Scrap, 426 U. S., at 810. The effect on interstate trade is the same as if the state legislature had imposed the policy on private cement producers. The Commerce Clause prohibits this severe restraint on commerce.

Ill

I share the Court’s desire to preserve state sovereignty. But the Commerce Clause long has been recognized as a limitation on that sovereignty, consciously designed to maintain a national market and defeat economic provincialism. The Court today approves protectionist state policies. In the absence of contrary congressional action,5 those policies now can be implemented as long as the State itself directly participates in the market.6

By enforcing the Commerce Clause in this case, the Court would work no unfairness on the people of South Dakota. They still could reserve cement for public projects and share in whatever return the plant generated. They could not, how*454ever, use the power of the State to furnish themselves with cement forbidden to the people of neighboring States.

The creation of a free national economy was a major goal of the States when they resolved to unite under the Federal Constitution. The decision today cannot be reconciled with that purpose.

*392Justice Kennedy

delivered the opinion of the Court.

To address pressing issues related to the large number of aliens within its borders who do not have a lawful right to *393be in this country, the State of Arizona in 2010 enacted a statute called the Support Our Law Enforcement and Safe Neighborhoods Act. The law is often referred to as S. B. 1070, the version introduced in the State Senate. See also H. B. 2162, 49th Leg., 2d Reg. Sess. (2010) (amending S. B. 1070). Its stated purpose is to “discourage and deter the unlawful entry and presence of aliens and economic activity by persons unlawfully present in the United States.” Note following Ariz. Rev. Stat. Ann. § 11-1051 (West 2012). The law’s provisions establish an official state policy of “attrition through enforcement.” Ibid. The question before the Court is whether federal law pre-empts and renders invalid four separate provisions of the state law.

I

The United States filed this suit against Arizona, seeking to enjoin S. B. 1070 as pre-empted.- Four provisions of the law are at issue here. Two create new state offenses. Section 3 makes failure to comply with federal alien-registration requirements a state misdemeanor. Ariz. Rev. Stat. Ann. § 13-1509 (West Supp. 2011). Section 5, in relevant part, *394makes it a misdemeanor for an unauthorized alien to seek or engage in work in the State; this provision is referred to as § 5(C). See § 13-2928(C). Two other provisions give specific arrest authority and investigative duties with respect to certain aliens to state and local law enforcement officers. Section 6 authorizes officers to arrest without a warrant a person “the officer has probable cause to believe . . . has committed any public offense that makes the person removable from the United States.” § 13-3883(A)(5). Section 2(B) provides that officers who conduct a stop, detention, or arrest must in some circumstances make efforts to verify the person’s immigration status with the Federal Government. See § 11-1051(B) (West 2012).

The United States District Court for the District of Arizona issued a preliminary injunction preventing the four provisions at issue from taking effect. 703 F. Supp. 2d 980, 1008 (2010). The Court of Appeals for the Ninth Circuit affirmed. 641 F. 3d 339, 366 (2011). It agreed that the United States had established a likelihood of success on its pre-emption claims. The Court of Appeals was unanimous in its conclusion that §§ 3 and 5(C) were likely pre-empted. Judge Bea dissented from the decision to uphold the preliminary injunction against §§ 2(B) and 6. This Court granted certiorari to resolve important questions concerning the interaction of state and federal power with respect to the law of immigration and alien status. 565 U. S. 1092 (2011).

nH I

A

The Government of the United States has broad, undoubted power over the subject of immigration and the status of aliens. See Toll v. Moreno, 458 U. S. 1, 10 (1982); see generally S. Legomsky & C. Rodriguez, Immigration and Refugee Law and Policy 115-132 (5th ed. 2009). This authority rests, in part, on the National Government’s constitutional power to “establish an uniform Rule of Naturaliza*395tion,” Art. I, § 8, cl. 4, and its inherent power as sovereign to control and conduct relations with foreign nations, see Toll, supra, at 10 (citing United States v. Curtiss-Wright Export Corp., 299 U. S. 304, 318 (1936)).

The federal power to determine immigration policy is well settled. Immigration policy can affect trade, investment, tourism, and diplomatic relations for the entire Nation, as well as the perceptions and expectations of aliens in this country who seek the full protection of its laws. See, e. g., Brief for United Mexican States as Amicus Curiae; see also Harisiades v. Shaughnessy, 342 U. S. 580, 588-589 (1952). Perceived mistreatment of aliens in the United States may lead to harmful reciprocal treatment of American citizens abroad. See Brief for Madeleine K. Albright et al. as Amici Curiae 24-30.

It is fundamental that foreign countries concerned about the status, safety, and security of their nationals in the United States must be able to confer and communicate on this subject with one national sovereign, not the 50 separate States. See Chy Lung v. Freeman, 92 U. S. 275, 279-280 (1876); see also The Federalist No. 3, p. 39 (C. Rossiter ed. 2003) (J. Jay) (observing that federal power would be necessary in part because “bordering States ... under the impulse of sudden irritation, and a quick sense of apparent interest or injury” might take action that would undermine foreign relations). This Court has reaffirmed that “[o]ne of the most important and delicate of all international relationships . . . has to do with the protection of the just rights of a country’s own nationals when those nationals are in another country.” Hines v. Davidowitz, 312 U. S. 52, 64 (1941).

Federal governance of immigration and alien status is extensive and complex. Congress has specified categories of aliens who may not be admitted to the United States. See 8 U. S. C. § 1182. Unlawful entry and unlawful reentry into the country are federal offenses. §§ 1325, 1326. Once here, aliens are required to register with the Federal Government *396and to carry proof of status on their person. See §§ 1301-1306. Failure to do so is a federal misdemeanor. §§ 1304(e), 1306(a). Federal law also authorizes States to deny nonciti-zens a range of public benefits, § 1622; and it imposes sanctions on employers who hire unauthorized workers, § 1324a.

Congress has specified which aliens may be removed from the United States and the procedures for doing so. Aliens may be removed if they were inadmissible at the time of entry, have been convicted of certain crimes, or meet other criteria set by federal law. See § 1227. Removal is a civil, not criminal, matter. A principal feature of the removal system is the broad discretion exercised by immigration officials. See Brief for Former Commissioners of the United States Immigration and Naturalization Service as Amici Curiae 8-13 (hereinafter Brief for Former INS Commissioners). Federal officials, as an initial matter, must decide whether it makes sense to pursue removal at all. If removal proceedings commence, aliens may seek asylum and other discretionary relief allowing them to remain in the country or at least to leave without formal removal. See § 1229a(c)(4); see also, e.g., §§ 1158 (asylum), 1229b (cancellation of removal), 1229c (voluntary departure).

Discretion in the enforcement of immigration law embraces immediate human concerns. Unauthorized workers trying to support their families, for example, likely pose less danger than alien smugglers or aliens who commit a serious crime. The equities of an individual ease may turn on many factors, including whether the alien has children born in the United States, long ties to the community, or a record of distinguished military service. Some discretionary decisions involve policy choices that bear on this Nation’s international relations. Returning an alien tq his own country may be deemed inappropriate even where he has committed a removable offense or fails to meet the criteria for admission. The foreign state may be mired in civil war, complicit in political persecution, or enduring conditions that create a *397real risk that the alien or his family will be harmed upon return. The dynamic nature of relations with other countries requires the Executive Branch to ensure that enforcement policies are consistent with this Nation’s foreign policy with respect to these and other realities.

Agencies in the Department of Homeland Security play a major role in enforcing the country’s immigration laws. United States Customs and Border Protection (CBP) is responsible for determining the admissibility of aliens and securing the country’s borders. See Dept, of Homeland Security, Office of Immigration Statistics, Immigration Enforcement Actions: 2010, p. 1 (2011). In 2010, CBP’s Border Patrol apprehended almost half a million people. Id., at 3. Immigration and Customs Enforcement (ICE), a second agency, “conducts criminal investigations involving the enforcement of immigration-related statutes.” Id., at 2. ICE also operates the Law Enforcement Support Center. LESC, as the Center is known, provides immigration status information to federal, state, and local officials around the clock. See App. 91. ICE officers are responsible “for the identification, apprehension, and removal of illegal aliens from the United States.” Immigration Enforcement Actions, at 2. Hundreds of thousands of aliens are removed by the Federal Government every year. See id., at 4 (reporting there were 387,242 removals, and 476,405 returns without a removal order, in 2010).

B

The pervasiveness of federal regulation does not diminish the importance of immigration policy to the States. Arizona bears many of the consequences of unlawful immigration. Hundreds of thousands of deportable aliens are apprehended in Arizona each year. Dept, of Homeland Security, Office of Immigration Statistics, 2010 Yearbook of Immigration Statistics 93 (2011) (Table 35). Unauthorized aliens who remain in the State constitute, by one estimate, almost 6% of the population. See J. Passel & D. Cohn, Pew Hispanic Center, *398U. S. Unauthorized Immigration Flows Are Down Sharply Since Mid-Decade 3 (2010). And in the State’s most populous county, these aliens are reported to be responsible for a disproportionate share of serious crime. See, e. g., S. Camarota & J. Vaughan, Center for Immigration Studies, Immigration and Crime: Assessing a Conflicted Issue 16 (2009) (Table 3) (estimating that unauthorized aliens constitute 8.9% of the population and are responsible for 21.8% of the felonies in Maricopa County, which includes Phoenix).

Statistics alone do not capture the full extent of Arizona’s concerns. Accounts in the record suggest there is an “epidemic of crime, safety risks, serious property damage, and environmental problems” associated with the influx of illegal migration across private land near the Mexican border. Brief for Petitioners 6. Phoenix is a major city of the United States, yet signs along an interstate highway 30 miles to the south warn the public to stay away. One reads, “DANGER—PUBLIC WARNING—TRAVEL NOT RECOMMENDED / Active Drug and Human Smuggling Area / Visitors May Encounter Armed Criminals and Smuggling Vehicles Traveling at High Rates of Speed.” App. 170 (punctuation altered); see also Brief for Petitioners 5-6. The problems posed to the State by illegal immigration must not be underestimated.

These concerns are the background for the formal legal analysis that follows. The issue is whether, under preemption principles, federal law permits Arizona to implement the state-law provisions in dispute.

J-H b-i I—I

Federalism, central to the constitutional design, adopts the principle that both the National and State Governments have elements of sovereignty the other is bound to respect. See Gregory v. Ashcroft, 501 U. S. 452, 457 (1991); U. S. Term Limits, Inc. v. Thornton, 514 U. S. 779, 838 (1995) (Kennedy, J., concurring). From the existence of two sovereigns fol*399lows the possibility that laws can be in conflict or at cross-purposes. The Supremacy Clause provides a clear rule that federal law “shall be the supreme Law of the Land; and the Judges in every state shall be bound thereby, any Thing in the Constitution or Laws of any state to the Contrary notwithstanding.” Art. VI, cl. 2. Under this principle, Congress has the power to pre-empt state law. See Crosby v. National Foreign Trade Council, 530 U. S. 363, 372 (2000); Gibbons v. Ogden, 9 Wheat. 1, 210-211 (1824). There is no doubt that Congress may withdraw specified powers from the States by enacting a statute containing an express preemption provision. See, e. g., Chamber of Commerce of United States of America v. Whiting, 563 U. S. 582, 592 (2011).

State law must also give way to federal law in at least two other circumstances. First, the States are precluded from regulating conduct in a field that Congress, acting within its proper authority, has determined must be regulated by its exclusive governance. See Gade v. National Solid Wastes Management Assn., 505 U. S. 88, 115 (1992) (Souter, J., dissenting). The intent to displace state law altogether can be inferred from a framework of regulation “so pervasive . . . that Congress left no room for the States to supplement it” or where there is a “federal interest ... so dominant that the federal system will be assumed to preclude enforcement of state laws on the same subject.” Rice v. Santa Fe Elevator Corp., 331 U. S. 218, 230 (1947); see English v. General Elec. Co., 496 U. S. 72, 79 (1990).

Second, state laws are pre-empted when they conflict with federal law. Crosby, supra, at 372. This includes cases where “compliance with both federal and state regulations is a physical impossibility,” Florida Lime & Avocado Growers, Inc. v. Paul, 373 U. S. 132, 142-143 (1963), and those instances where the challenged state law “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress,” Hines, 312 U. S., at 67; see also *400Crosby, supra, at 373 (“What is a sufficient obstacle is a matter of judgment, to be informed by examining the federal statute as a whole and identifying its purpose and intended effects”)- In pre-emption analysis, courts should assume that “the historic police powers of the States” are not superseded “unless that was the clear and manifest purpose of Congress.” Rice, supra, at 230; see Wyeth v. Levine, 555 U. S. 555, 565 (2009).

The four challenged provisions of the state law each must be examined under these pre-emption principles.

IV

A

Section S

Section 3 of S. B. 1070 creates a new state misdemeanor. It forbids the “willful failure to complete or carry an alien registration document ... in violation of 8 United States Code § 1304(e) or 1306(a).” Ariz. Rev. Stat. Ann. § 13— 1509(A). In effect, § 3 adds a state-law penalty for conduct proscribed by federal law. The United States contends that this state enforcement mechanism intrudes on the field of alien registration, a field in which Congress has left no room for States to regulate. See Brief for United States 27, 31.

The Court discussed federal alien-registration requirements in Hines, supra. In 1940, as international conflict spread, Congress added to federal immigration law a “complete system for alien registration.” Id., at 70. The new federal law struck a careful balance. It punished an alien’s willful failure to register but did not require aliens to carry identification cards. There were also limits on the sharing of registration records and fingerprints. The Court found that Congress intended the federal plan for registration to be a “single integrated and all-embracing system.” Id., at 74. Because this “complete scheme ... for the registration of aliens” touched on foreign relations, it did not allow the States to “curtail or complement” federal law or to “enforce *401additional or auxiliary regulations.” Id., at 66-67. As a consequence, the Court ruled that Pennsylvania could not enforce its own alien-registration program. See id., at 59, 74.

The present regime of federal regulation is not identical to the statutory framework considered in Hines, but it remains comprehensive. Federal law now includes a requirement that aliens carry proof of registration. 8 U. S. C. § 1804(e). Other aspects, however, have stayed the same. Aliens who remain in the country for more than 30 days must apply for registration and be fingerprinted. Compare § 1302(a) with § 452(a) (1940 ed.). Detailed information is required, and any change of address has to be reported to the Federal Government. Compare §§ 1304(a), 1305(a) (2006 ed.) with §§ 455(a), 456 (1940 ed.). The statute continues to provide penalties for the willful failure to register. Compare § 1306(a) (2006 ed.) with §457 (1940 ed.).

The framework enacted by Congress leads to the conclusion here, as it did in Hines, that the Federal Government has occupied the field of alien registration. See American Ins. Assn. v. Garamendi, 539 U. S. 396, 419, n. 11 (2003) (characterizing Hines as a field pre-emption case); Pennsylvania v. Nelson, 350 U. S. 497, 504 (1956) (same); see also Dinh, Reassessing the Law of Preemption, 88 Geo. L. J. 2085, 2098-2099, 2107 (2000) (same). The federal statutory directives provide a full set of standards governing alien registration, including the punishment for noncompliance. It was designed as a “‘harmonious whole.’” Hines, supra, at 72. Where Congress occupies an entire field, as it has in the field of alien registration, even complementary state regulation is impermissible. Field pre-emption reflects a congressional decision to foreclose any state regulation in the area, even if it is parallel to federal standards. See Silkwood v. Kerr-McGee Corp., 464 U. S. 238, 249 (1984).

Federal law makes a single sovereign responsible for maintaining a comprehensive and unified system to keep track of *402aliens within the Nation’s borders. If §3 of the Arizona statute were valid, every State could give itself independent authority to prosecute federal registration violations, “diminish[ing] the [Federal Government] ⅛ control over enforcement” and “detracting] from the ‘integrated scheme of regulation’ created by Congress.” Wisconsin Dept. of Industry v. Gould Inc., 475 U. S. 282, 288-289 (1986). Even if a State may make violation of federal law a crime in some instances, it cannot do so in a field (like the field of alien registration) that has been occupied by federal law. See California v. Zook, 336 U. S. 725, 730-731, 733 (1949); see also In re Loney, 134 U. S. 372, 375-376 (1890) (States may not impose their own punishment for perjury in federal courts).

Arizona contends that § 3 can survive pre-emption because the provision has the same aim as federal law and adopts its substantive standards. This argument not only ignores the basic premise of field pre-emption—that States may not enter, in any respect, an area the Federal Government has reserved for itself—but also is unpersuasive on its own terms. Permitting the State to impose its own penalties for the federal offenses here would conflict with the careful framework Congress adopted. Cf. Buckman Co. v. Plaintiffs’ Legal Comm., 531 U. S. 341, 347-348 (2001) (States may not impose their own punishment for fraud on the Food and Drug Administration); Wisconsin Dept., supra, at 288 (States may not impose their own punishment for repeat violations of the National Labor Relations Act). Were §3 to come into force, the State would have the power to bring criminal charges against individuals for violating a federal law even in circumstances where federal officials in charge of the comprehensive scheme determine that prosecution would fi-ustrate federal policies.

There is a further intrusion upon the federal scheme. Even where federal authorities believe prosecution is appropriate, there is an inconsistency between § 3 and federal law *403with respect to penalties. Under federal law, the failure to carry registration papers is a misdemeanor that may be punished by a fine, imprisonment, or a term of probation. See 8 U. S. C. § 1304(e) (2006 ed.); 18 U. S. C. § 3561. State law, by contrast, rules out probation as a possible sentence (and also eliminates the possibility of a pardon). See Ariz. Rev. Stat. Ann. § 13-1509(D). This state framework of sanctions creates a conflict with the plan Congress put in place. See Wisconsin Dept., supra, at 286 (“[C]onflict is imminent whenever two separate remedies are brought to bear on the same activity” (internal quotation marks omitted)).

These specific conflicts between state and federal law simply underscore the reason for field pre-emption. As it did in Hines, the Court now concludes that, with respect to the subject of alien registration, Congress intended to preclude States from “complement[ing] the federal law, or enforcing] additional or auxiliary regulations.” 312 U. S., at 66-67. Section 3 is pre-empted by federal law.

B

Section 5(C)

Unlike §3, which replicates federal statutory requirements, §5(C) enacts a state criminal prohibition where no federal counterpart exists. The provision makes it a state misdemeanor for “an unauthorized alien to knowingly apply for work, solicit work in a public place or perform work as an employee or independent contractor” in Arizona. Ariz. Rev. Stat. Ann. §13-2928(C). Violations can be punished by a $2,500 fine and incarceration for up to six months. See § 13-2928(F); see also §§ 13-707(A)(1) (West 2010); 13-802(A); 13-902(A)(5) (West Supp. 2011). The United States contends that the provision upsets the balance struck by the Immigration Reform and Control Act of 1986 (IRCA) and must be pre-empted as an obstacle to the federal plan of regulation and control.

*404When there was no comprehensive federal program regulating the employment of unauthorized aliens, this Court found that a State had authority to pass its own laws on the subject. In 1971, for example, California passed a law imposing civil penalties on the employment of aliens who were “not entitled to lawful residence in the United States if such employment would have an adverse effect on lawful resident workers.” 1971 Cal. Stats. ch. 1442, § 1(a). The law was upheld against a pre-emption challenge in De Canas v. Bica, 424 U. S. 351 (1976). De Canas recognized that “States possess broad authority under their police powers to regulate the employment relationship to protect workers within the State.” Id., at 356. At that point, however, the Federal Government had expressed no more than “a peripheral concern with [the] employment of illegal entrants.” Id., at 360; see Whiting, 563 U. S., at 588.

Current federal law is substantially different from the regime that prevailed when De Canas was decided. Congress enacted IRCA as a comprehensive framework for “combating the employment of illegal aliens.” Hoffman Plastic Compounds, Inc. v. NLRB, 535 U. S. 137, 147 (2002). The law makes it illegal for employers to knowingly hire, recruit, refer, or continue to employ unauthorized workers. See 8 U. S. C. §§ 1324a(a)(1)(A), (a)(2). It also requires every employer to verify the employment authorization status of prospective employees. See §§ 1324a(a)(1)(B), (b); 8 CFR § 274a.2(b) (2012). These requirements are enforced through criminal penalties and an escalating series of civil penalties tied to the number of times an employer has violated the provisions. See 8 U. S. C. §§ 1324a(e)(4), (f); 8 CFR § 274a.10.

This comprehensive framework does not impose federal criminal sanctions on the employee side (i. e., penalties on aliens who seek or engage in unauthorized work). Under federal law some civil penalties are imposed instead. With certain exceptions, aliens who accept unlawful employment *405are not eligible to have their status adjusted to that of a lawful permanent resident. See 8 U. S. C. §§ 1255(c)(2), (c)(8). Aliens also may be removed from the country for having engaged in unauthorized work. See § 1227(a)(1) (C)(i); 8 CFR § 214.1(e). In addition to specifying these civil consequences, federal law makes it a crime for unauthorized workers to obtain employment through fraudulent means. See 18 U. S. C. § 1546(b). Congress has made clear, however, that any information employees submit to indicate their work status “may not be used” for purposes other than prosecution under specified federal criminal statutes for fraud, perjury, and related conduct. See 8 U. S. C. §§ 1324a(b)(5), (d)(2)(F)-(G).

The legislative background of IRCA underscores the fact that Congress made a deliberate choice not to impose criminal penalties on aliens who seek, or engage in, unauthorized employment. A commission established by Congress to study immigration policy and to make recommendations concluded these penalties would be “unnecessary and unworkable.” U. S. Immigration Policy and the National Interest: The Final Report and Recommendations of the Select Commission on Immigration and Refugee Policy With Supplemental Views by Commissioners 65-66 (1981); see § 4, 92 Stat. 907. Proposals to make unauthorized work a criminal offense were debated and discussed during the long process of drafting IRCA. See Brief for Service Employees International Union et al. as Amici Curiae 9-12. But Congress rejected them. See, e. g., 119 Cong. Rec. 14184 (1973) (statement of Rep. Dennis). In the end, IRCA’s framework reflects a considered judgment that making criminals out of aliens engaged in unauthorized work—aliens who already face the possibility of employer exploitation because of their removable status—would be inconsistent with federal policy and objectives. See, e. g., Hearings before Subcommittee No. 1 of the House Committee on the Judiciary, 92d Cong., 1st Sess., pt. 3, pp. 919-920 (1972) (statement of *406Rep. Rodino, the eventual sponsor of IRCA in the House of Representatives).

IRCA’s express pre-emption provision, which in most instances bars States from imposing penalties on employers of unauthorized aliens, is silent about whether additional penalties may be imposed against the employees themselves. See 8 U.S.C. § 1324a(h)(2); Whiting, supra, at 587-588. But the existence of an “express pre-emption provisio[n] does not bar the ordinary working of conflict pre-emption principles” or impose a “ ‘special burden’ ” that would make it more difficult to establish the pre-emption of laws falling outside the clause. Geier v. American Honda Motor Co., 529 U. S. 861, 869-872 (2000); see Sprietsma v. Mercury Marine, 537 U. S. 51, 65 (2002).

The ordinary principles of pre-emption include the well-settled proposition that a state law is pre-empted where it “stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.” Hines, 312 U. S., at 67. Under § 5(C) of S. B. 1070, Arizona law would interfere with the careful balance struck by Congress with respect to unauthorized employment of aliens. Although § 5(C) attempts to achieve one of the same goals as federal law—the deterrence of unlawful employment—it involves a conflict in the method of enforcement. The Court has recognized that a “[cjonflict in technique can be fully as disruptive to the system Congress erected as conflict in overt policy.” Motor Coach Employees v. Lockridge, 403 U. S. 274, 287 (1971). The correct instruction to draw from the text, structure, and history of IRCA is that Congress decided it would be inappropriate to impose criminal penalties on aliens who seek or engage in unauthorized employment. It follows that a state law to the contrary is an obstacle to the regulatory system Congress chose. See Puerto Rico Dept. of Consumer Affairs v. ISLA Petroleum Corp., 485 U. S. 495, 503 (1988) (“Where a comprehensive federal scheme intentionally leaves a portion of the regulated field without controls, then *407the pre-emptive inference can be drawn—not from federal inaction alone, but from inaction joined with action”). Section 5(C) is pre-empted by federal law.

C

Section 6

Section 6 of S. B. 1070 provides that a state officer, “without a warrant, may arrest a person if the officer has probable cause to believe . . . [the person] has committed any public offense that makes [him] removable from the United States.” Ariz. Rev. Stat. Ann. § 13-3883(A)(5). The United States argues that arrests authorized by this statute would be an obstacle to the removal system Congress created.

As a general rule, it is not a crime for a removable alien to remain present in the United States. See INS v. Lopez-Mendoza, 468 U. S. 1032, 1038 (1984). If the police stop someone based on nothing more than possible removability, the usual predicate for an arrest is absent. When an alien is suspected of being removable, a federal official issues an administrative document called a “Notice to Appear.” See 8 U. S. C. § 1229(a); 8 CFR § 239.1(a). The form does not authorize an arrest. Instead, it gives the alien information about the proceedings, including the time and date of the removal hearing. See 8 U. S. C. § 1229(a)(1). If an alien fails to appear, an in absentia order may direct removal. § 1229a(b)(5)(A).

The federal statutory structure instructs when it is appropriate to arrest an alien during the removal process. For example, the Attorney General can exercise discretion to issue a warrant for an alien’s arrest and detention “pending a decision on whether the alien is to be removed from the United States.” § 1226(a); see Memorandum from John Morton, Director, ICE, to All Field Office Directors et al., Exercising Prosecutorial Discretion Consistent With the Civil Immigration Enforcement Priorities of the Agency for the Apprehension, Detention, and Removal of Aliens (June 17, *4082011) (hereinafter 2011 ICE Memorandum) (describing factors informing this and related decisions). And if an alien is ordered removed after a hearing, the Attorney General will issue a warrant. See 8 CFR § 241.2(a)(1). In both instances, the warrants are executed by federal officers who have received training in the enforcement of immigration law. See §§ 241.2(b), 287.5(e)(3). If no federal warrant has been issued, those officers have more limited authority. See 8 U. S. C. § 1357(a). They may arrest an alien for being “in the United States in violation of any [immigration] law or regulation,” for example, but only where the alien “is likely to escape before a warrant can be obtained.” § 1357(a)(2).

Section 6 attempts to provide state officers even greater authority to arrest aliens on the basis of possible removability than Congress has given to trained federal immigration officers. Under state law, officers who believe an alien is removable by reason of some “public offense” would have the power to conduct an arrest on that basis regardless of whether a federal warrant has issued or the alien is likely to escape. This state authority could be exercised without any input from the Federal Government about whether an arrest is warranted in a particular case. This would allow the State to achieve its own immigration policy. The result could be unnecessary harassment of some aliens (for instance, a veteran, college student, or someone assisting with a criminal investigation) who federal officials determine should not be removed.

This is not the system Congress created. Federal law specifies limited circumstances in which state officers may perform the functions of an immigration officer. A principal example is when the Attorney General has granted that authority to specific officers in a formal agreement with a state or local government. See § 1357(g)(1); see also § 1103(a)(10) (authority may be extended in the event of an “imminent mass influx of aliens arriving off the coast of the United *409States”); § 1252c (authority to arrest in specific circumstance after consultation with the Federal Government); § 1324(c) (authority to arrest for bringing in and harboring certain aliens). Officers covered by these agreements are subject to the Attorney General’s direction and supervision. § 1357(g)(3). There are significant complexities involved in enforcing federal immigration law, including the determination whether a person is removable. See Padilla v. Kentucky, 559 U. S. 356, 379-380 (2010) (Alito, J., concurring in judgment). As a result, the agreements reached with the Attorney General must contain written certification that officers have received adequate training to carry out the duties of an immigration officer. See 11357(g)(2); cf. 8 CFR §§ 287.5(c) (arrest power contingent on training), 287.1(g) (defining the training).

By authorizing state officers to decide whether an alien should be detained for being removable, § 6 violates the principle that the removal process is entrusted to the discretion of the Federal Government. See, e. g., Reno v. American-Arab Anti-Discrimination Comm., 525 U. S. 471, 483-484 (1999); see also Brief for Former INS Commissioners 8-13. A decision on removability requires a determination whether it is appropriate to allow a foreign national to continue living in the United States. Decisions of this nature touch on foreign relations and must be made with one voice. See Jama v. Immigration and Customs Enforcement, 543 U. S. 335, 348 (2005) (“Removal decisions, including the selection of a removed alien’s destination, may implicate [the Nation’s] relations with foreign powers and require consideration of changing political and economic circumstances” (internal quotation marks omitted)); see also Galvan v. Press, 347 U. S. 522, 531 (1954) (“Policies pertaining to the entry of aliens and their right to remain here are . . . entrusted exclusively to Congress . . . ”); Truax v. Raich, 239 U. S. 33, 42 (1915) (“The authority to control immigration—to *410admit or exclude aliens—is vested solely in the Federal Government”).

In defense of § 6, Arizona notes a federal statute permitting state officers to “cooperate with the Attorney General in the identification, apprehension, detention, or removal of aliens not lawfully present in the United States.” 8 U. S. C. § 1357(g)(10)(B). There may be some ambiguity as to what constitutes cooperation under the federal law; but no coherent understanding of the term would incorporate the unilateral decision of state officers to arrest an alien for being removable absent any request, approval, or other instruction from the Federal Government. The Department of Homeland Security gives examples of what would constitute cooperation under federal law. These include situations where States participate in a joint task force with federal officers, provide operational support in executing a warrant, or allow federal immigration officials to gain access to detainees held in state facilities. See Dept,3 of Homeland Security, Guidance on State and Local Governments’ Assistance in Immigration Enforcement and Related Matters 13-14 (2011), online at http://www.dhs.gov/files/resources/immigration.shtm (all Internet materials as visited June 21, 2012, and available in Clerk of Court’s case file). State officials can also assist the Federal Government by responding to requests for information about when an alien will be released from their custody. See § 1357(d). But the unilateral state action to detain authorized by §6 goes far beyond these measures, defeating any need for real cooperation.

Congress has put in place a system in which state officers may not make warrantless arrests of aliens based on possible removability except in specific, limited circumstances. By nonetheless authorizing state and local officers to engage in these enforcement activities as a general matter, § 6 creates an obstacle to the full purposes and objectives of Congress. See Hines, 312 U. S., at 67. Section 6 is pre-empted by federal law.

*411D

Section 2(B)

Section 2(B) of S. B. 1070 requires state officers to make a “reasonable attempt ... to' determine the immigration status” of any person they stop, detain, or arrest on some other legitimate basis if “reasonable suspicion exists that the person is an alien and is unlawfully present in the United States.” Ariz. Rev. Stat. Ann. § 11-1051(B). The law also provides that “[a]ny person who is arrested shall have the person’s immigration status determined before the person is released.” Ibid. The accepted way to perform these status checks is to contact ICE, which maintains a database of immigration records.

Three limits are built into the state provision. First, a detainee is presumed not to be an alien unlawfully present in the United States if he or she provides a valid Arizona driver’s license or similar identification. Second, officers “may not consider race, color or national origin . . . except to the extent permitted by the United States [and] Arizona Constitution^].” Ibid. Third, the provision must be “implemented in a manner consistent with federal laws regulating immigration, protecting the civil rights of all persons and respecting the privileges and immunities of United States citizens.” §11-1051(L).

The United States and its amici contend that, even with these limits, the State’s verification requirements pose an obstacle to the framework Congress put in place. The first concern is the mandatory nature of the status checks. The second is the possibility of prolonged detention while the checks are being performed.

1

Consultation between federal and state officials is an important feature of the immigration system. Congress has made clear that no formal agreement or special training needs to be in place for state officers to “communicate with *412the [Federal Government] regarding the immigration status of any individual, including reporting knowledge that a particular alien is not lawfully present in the United States.” 8 U. S. C. § 1357(g)(10)(A). And Congress has obligated ICE to respond to any request made by state officials for verification of a person’s citizenship or immigration status. See § 1373(c); see also § 1226(d)(1)(A) (requiring a system for determining whether individuals arrested for aggravated felonies are aliens). ICE’s Law Enforcement Support Center operates “24 hours a day, seven days a week, 365 days a year” and provides, among other things, “immigration status, identity information and real-time assistance to local, state and federal law enforcement agencies.” ICE, Fact Sheet: Law Enforcement Support Center (May 29, 2012), online at http://www.iee.gov/news/library/factsheets/lesc.htm. LESC responded to more than 1 million requests for information in 2009 alone. App. 93.

The United States argues that making status verification mandatory interferes with the federal immigration scheme. It is true that §2(B) does not allow state officers to consider federal enforcement priorities in deciding whether to contact ICE about someone they have detained. See Brief for United States 47-50. In other words, the officers must make an inquiry even in cases where it seems unlikely that the Attorney General would have the alien removed. This might be the case, for example, when an alien is an elderly veteran with significant and longstanding ties to the community. See 2011 ICE Memorandum 4-5 (mentioning these factors as relevant).

Congress has done nothing to suggest it is inappropriate to communicate with ICE in these situations, however. Indeed, it has encouraged the sharing of information about possible immigration violations. See 8 U. S. C. § 1357(g) (10)(A). A federal statute regulating the public benefits provided to qualified aliens in fact instructs that “no State or local government entity may be prohibited, or in any way *413restricted, from sending to or receiving from [ICE] information regarding the immigration status, lawful or unlawful, of an alien in the United States.” § 1644. The federal scheme thus leaves room for a policy requiring state officials to contact ICE as a routine matter. Cf. Whiting, 563 U. S., at 609-610 (rejecting argument that federal law pre-empted Arizona’s requirement that employers determine whether employees were eligible to work through the federal E-Verify system where the Federal Government had encouraged its use).

2

Some who support the challenge to §2(B) argue that, in practice, state officers will be required to delay the release of some detainees for no reason other than to verify their immigration status. See, e. g., Brief for Former Arizona Attorney General Terry Goddard et al. as Amici Curiae 37, n. 49. Detaining individuals solely to verify their immigration status would raise constitutional concerns. See, e. g., Arizona v. Johnson, 555 U. S. 323, 333 (2009); Illinois v. Caballes, 543 U. S. 405, 407 (2005) (“A seizure that is justified solely by the interest in issuing a warning ticket to the driver can become unlawful if it is prolonged beyond the time reasonably required to complete that mission”). And it would disrupt the federal framework to put state officers in the position of holding aliens in custody for possible unlawful presence without federal direction and supervision. Cf. Part IV-C, supra (concluding that Arizona may not authorize warrantless arrests on. the basis of removability). The program put in place by Congress does not allow state or local officers to adopt this enforcement mechanism.

But § 2(B) could be read to avoid these concerns. To take one example, a person might be stopped for jaywalking in Tucson and be unable to produce identification. The first sentence of §2(B) instructs officers to make a “reasonable” attempt to verify his immigration status with ICE if there is reasonable suspicion that his presence in the United States *414is unlawful. The state courts may conclude that, unless the person continues to be suspected of some crime for which he may be detained by state officers, it would not be reasonable to prolong the stop for the immigration inquiry. See Reply Brief 12, n. 4 (“[Section 2(B)] does not require the verification be completed during the stop or detention if that is not reasonable or practicable”); cf. Muehler v. Mena, 544 U. S. 93, 101 (2005) (finding no Fourth Amendment violation where questioning about immigration status did not prolong a stop).

To take another example, a person might be held pending release on a charge of driving under the influence of alcohol. As this goes beyond a mere stop, the arrestee (unlike the jaywalker) would appear to be subject to the categorical requirement in the second sentence of § 2(B) that “[a]ny person who is arrested shall have the person’s immigration status determined before [he] is released.” State courts may read this as an instruction to initiate a status check every time someone is arrested, or in some subset of those cases, rather than as a command to hold the person until the check is complete no matter the circumstances. Even if the law is read as an instruction to complete a check while the person is in custody, moreover, it is not clear at this stage and on this record that the verification process would result in prolonged detention.

However the law is interpreted, if §2(B) only requires state officers to conduct a status check during the course of an authorized, lawful detention or after a detainee has been released, the provision likely would survive pre-emption—at least absent some showing that it has other consequences that are adverse to federal law and its objectives. There is no need in this case to address whether reasonable suspicion of illegal entry or another immigration crime would be a legitimate basis for prolonging a detention, or whether this too would be pre-empted by federal law. See, e. g., United States v. Di Re, 332 U. S. 581, 589 (1948) (authority of state officers to make arrests for federal crimes is, absent federal statutory instruction, a matter of state law); Gonzales v. Peo*415ria, 722 F. 2d 468, 475-476 (CA9 1983) (concluding that Arizona officers have authority to enforce the criminal provisions of federal immigration law), overruled on other grounds in Hodgers-Durgin v. de la Vina, 199 F. 3d 1037 (CA9 1999).

The nature and timing of this case counsel caution in evaluating the validity of §2(B). The Federal Government has brought suit against a sovereign State to challenge the provision even before the law has gone into effect. There is a basic uncertainty about what the law means and how it will be enforced. At this stage, without the benefit of a definitive interpretation from the state courts, it would be inappropriate to assume §2(B) will be construed in a way that creates a conflict with federal law. Cf. Fox v. Washington, 236 U. S. 273, 277 (1915) (“So far as statutes fairly may be construed in such a way as to avoid doubtful constitutional questions they should be so construed; and it is to be presumed that state laws will be construed in that way by the state courts” (citation omitted)). As a result, the United States cannot prevail in its current challenge. See Huron Portland Cement Co. v. Detroit, 362 U. S. 440, 446 (1960) (“To hold otherwise would be to ignore the teaching of this Court’s decisions which enjoin seeking out conflicts between state and federal regulation where none clearly exists”). This opinion does not foreclose other pre-emption and constitutional challenges to the law as interpreted and applied after it goes into effect.

V

Immigration policy shapes the destiny of the Nation. On May 24, 2012, at one of this Nation’s most distinguished museums of history, a dozen immigrants stood before the tattered flag that inspired Francis Scott Key to write the National Anthem. There they took the oath to become American citizens. The Smithsonian, News Release, Smithsonian Citizenship Ceremony Welcomes a Dozen New Americans (May 24, 2012), online at http://newsdesk.si.edu/ releases. These naturalization ceremonies bring together *416men and women of different origins who now share a common destiny. They swear a common oath to renounce fidelity to foreign princes, to defend the Constitution, and to bear arms on behalf of the country when required by law. 8 CFR § 837.1(a). The history of the United States is in part made of the stories, talents, and lasting contributions of those who crossed oceans and deserts to come here.

The National Government has significant power to regulate immigration. With power comes responsibility, and the sound exercise of national power over immigration depends on the Nation’s meeting its responsibility to base its laws on a political will informed by searching, thoughtful, rational civic discourse. Arizona may have understandable frustrations with the problems caused by illegal immigration while that process continues, but the State may not pursue policies that undermine federal law.

* * *

The United States has established that §§3, 5(C), and 6 of S. B. 1070 are pre-empted. It was improper, however, to enjoin §2(B) before the state courts had an opportunity to construe it and without some showing that enforcement of the provision in fact conflicts with federal immigration law and its objectives.

The judgment of the Court of Appeals for the Ninth Circuit is affirmed in part and reversed in part. The case is remanded for further proceedings consistent with this opinion.

It is so ordered.

Justice Kagan took no part in the consideration or decision of this case.

Justice Scalia,

concurring in part and dissenting in part.

The United States is an indivisible “Union of sovereign States.” Hinderlider v. La Plata River & Cherry Creek Ditch Co., 304 U. S. 92, 104 (1938). Today’s opinion, approv*417ing virtually all of the Ninth Circuit’s injunction against enforcement of the four challenged provisions of Arizona’s law, deprives States of what most would consider the defining characteristic of sovereignty: the power to exclude from the sovereign’s territory people who have no right to be there. Neither the Constitution itself nor even any law passed by Congress supports this result. I dissent.

I—1

As a sovereign, Arizona has the inherent power to exclude persons from its territory, subject only to those limitations expressed in the Constitution or constitutionally imposed by Congress. That power to exclude has long been recognized as inherent in sovereignty. Emer de Vattel’s seminal 1758 treatise on the Law of Nations stated:

“The sovereign may forbid the entrance of his territory either to foreigners in general, or in particular cases, or to certain persons, or for certain particular purposes, according as he may think it advantageous to the state. There is nothing in all this, that does not flow from the rights of domain and sovereignty: every one is obliged to pay respect to the prohibition; and whoever dares to violate it, incurs the penalty decreed to render it effectual.” The Law of Nations, bk. II, ch. VII, § 94, p. 309 (B. Kapossy & R. Whatmore eds. 2008).

See also 1 R. Phillimore, Commentaries Upon International Law, pt. Ill, ch. X, *233 (“It is a received maxim of International Law, that the Government of a State may prohibit the entrance of strangers into the country”).1

*418There is no doubt that “before the adoption of the constitution of the United States” each State had the authority to “prevent [itself] from being burdened by an influx of persons.” Mayor of New York v. Miln, 11 Pet. 102, 132-133 (1837). And the Constitution did not strip the States of that authority. To the contrary, two of the Constitution’s provisions were designed to enable the States to prevent “the intrusion of obnoxious aliens through other States.” Letter from James Madison to Edmund Randolph (Aug. 27,1782), in 1 Writings of James Madison 226 (G. Hunt ed. 1900); accord, The Federalist No. 42, pp. 269-271 (C. Rossiter ed. 1961) (J. Madison). The Articles of Confederation had provided that “the free inhabitants of each of these States, paupers, vagabonds and fugitives from justice excepted, shall be entitled to all privileges and immunities of free citizens in the several States.” Art. IV. This meant that an unwelcome alien could obtain all the rights of a citizen of one State simply by first becoming an inhabitant of another. To remedy this, the Constitution’s Privileges and Immunities Clause provided that “[t]he Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several States.” Art. IV, §2, cl. 1 (emphasis added). But if one State had particularly lax citizenship standards, it might still serve as a gateway for the entry of “obnoxious aliens” into other States. This problem was solved “by authorizing the general government to establish a uniform rule of naturalization throughout the United States.” The Federalist No. 42, supra, at 271; see Art. I, § 8, cl. 4. In other words, the naturalization power was given to Congress not to abrogate States’ power to exclude those they did not want, but to vindicate it.

*419Two other provisions of the Constitution are an acknowledgment of the States’ sovereign interest in protecting their borders. Article I provides that “[n]o State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it’s inspection Laws.” § 10, cl. 2 (emphasis added). This assumed what everyone assumed: that the States could exclude from their territory dangerous or unwholesome goods. A later portion of the same section provides that “[n]o State shall, without the Consent of Congress, ... engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.” Cl. 3 (emphasis added). This limits the States’ sovereignty (in a way not relevant here) but leaves intact their inherent power to protect their territory.

Notwithstanding “[t]he myth of an era of unrestricted immigration” in the first 100 years of the Republic, the States enacted numerous laws restricting the immigration of certain classes of aliens, including convicted criminals, indigents, persons with contagious diseases, and (in Southern States) freed blacks. Neuman, The Lost Century of American Immigration Law (1776-1875), 93 Colum. L. Rev. 1833, 1835, 1841-1880 (1993). State laws not only provided for the removal of unwanted immigrants but also imposed penalties on unlawfully present aliens and those who aided their immigration.2 Id., at 1883.

In fact, the controversy surrounding the Alien and Sedition Acts involved a debate over whether, under the Constitution, the States had exclusive authority to enact such immigration laws. Criticism of the Sedition Act has become a prominent feature of our First Amendment jurisprudence, see, e. g., New York Times Co. v. Sullivan, 376 U. S. 254, *420273-276 (1964), but one of the Alien Acts3 also aroused controversy at the time:

“Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That it shall be lawful for the President of the United States at any time during the continuance of this act, to order all such aliens as he shall judge dangerous to the peace and safety of the United States, or shall have reasonable grounds to suspect are concerned in any treasonable or secret machinations against the government thereof, to depart out of the territory of the United States ...An Act concerning Aliens, 1 Stat. 570-571.

The Kentucky and Virginia Resolutions, written in denunciation of these Acts, insisted that the power to exclude unwanted aliens rested solely in the States. Jefferson’s Kentucky Resolutions insisted “that alien Mends are under the jurisdiction and protection of the laws of the state wherein they are [and] that no power over them has been delegated to the United States, nor prohibited to the individual states, distinct from their power over citizens.” Kentucky Resolutions of 1798, reprinted in J. Powell, Languages of Power: A Sourcebook of Early American Constitutional History 131 (1991). Madison’s Virginia Resolutions likewise contended that the Alien Act purported to give the President “a power nowhere delegated to the federal government.” Virginia Resolutions of 1798, in id., at 134 (emphasis deleted). Notably, moreover, the Federalist proponents of the Act defended it primarily on the ground that “[t]he removal of aliens is the usual preliminary of hostility” and could therefore be justified in exercise of the Federal Government’s war powers. Massachusetts Resolutions in Reply to Virginia, in id., at 136.

In Mayor of New York v. Miln, this Court considered a New York statute that required the commander of any ship *421arriving in New York from abroad to disclose “the name, place of birth, and last legal settlement, age and occupation . . . of all passengers . . . with the intention of proceeding to the said city.” 11 Pet., at 130-131. After discussing the sovereign authority to regulate the entrance of foreigners described by De Vattel, the Court said:

“The power ... of New York to pass this law having undeniably existed at the formation of the constitution, the simple inquiry is, whether by that instrument it was taken from the states, and granted to congress; for if it were not, it yet remains with them.” Id., at 132.

And the Court held that it remains. Id., at 139.

II

One would conclude from the foregoing that after the adoption of the Constitution there was some doubt about the power of the Federal Government to control immigration, but no doubt about the power of the States to do so. Since the founding era (though not immediately), doubt about the Federal Government’s power has disappeared. Indeed, primary responsibility for immigration policy has shifted from the States to the Federal Government. Congress exercised its power “[t]o establish an uniform Rule of Naturalization,” Art. I, §8, cl. 4, very early on, see Am Act to establish an uniform Rule of Naturalization, ch. 3,1 Stat. 103. But with the fleeting exception of the Alien Act, Congress did not enact any legislation regulating immigration for the better part of a century. In 1862, Congress passed “An Act to prohibit the ‘Coolie Trade’ by American Citizens in American Vessels,” which prohibited “procuring [Chinese nationals]... to be disposed of, or sold, or transferred, for any term of years or for any time whatever, as servants or apprentices, or to be held to service or labor.” Ch. 27, 12 Stat. 340. Then, in 1875, Congress amended that Act to bar admission to Chinese, Japanese, and other Asian immigrants who had *422“entered into a contract or agreement for a term of service within the United States, for lewd and immoral purposes.” An act supplementary to the acts in relation to immigration, ch. 141, 18 Stat. 477. And in 1882, Congress enacted the first general immigration statute. See An act to regulate Immigration, 22 Stat. 214. Of course, it hardly bears mention that federal immigration law is now extensive.

I accept that as a valid exercise of federal power—not because of the Naturalization Clause (it has no necessary connection to citizenship) but because it is an inherent attribute of sovereignty no less for the United States than for the States. As this Court has said, it is an “ ‘accepted maxim of international law, that every sovereign nation has the power, as inherent in sovereignty, and essential to self-preservation, to forbid the entrance of foreigners within its dominions.’” Fong Yue Ting v. United States, 149 U. S. 698, 705 (1893) (quoting Ekiu v. United States, 142 U. S. 651, 659 (1892)). That is why there was no need to set forth control of immigration as one of the enumerated powers of Congress, although an acknowledgment of that power (as well as of the States’ similar power, subject to federal abridgment) was contained in Art. I, § 9, which provided that “[t]he Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight . . . .”

In light of the predominance of federal immigration restrictions in modern times, it is easy to lose sight of the States' traditional role in regulating immigration—and to overlook their sovereign prerogative to do so. I accept as a given that state regulation is excluded by the Constitution when (1) it has been prohibited by a valid federal law, or (2) it conflicts with federal regulation—when, for example, it admits those whom federal regulation would exclude, or excludes those whom federal regulation would admit.

*423Possibility (1) need not be considered here: There is no federal law prohibiting the States’ sovereign power to exclude (assuming federal authority to enact such a law). The mere existence of federal action in the immigration area— and the so-called field pre-emption arising from that action, upon which the Court’s opinion so heavily relies, ante, at 401-403—cannot be regarded as such a prohibition. We are not talking here about a federal law prohibiting the States from regulating bubble-gum advertising, or even the construction of nuclear plants. We are talking about a federal law going to the core of state sovereignty: the power to exclude. Like elimination of the States’ other inherent sovereign power, immunity from suit, elimination of the States’ sovereign power to exclude requires that “Congress ... unequivocally expres[s] its intent to abrogate,” Seminole Tribe of Fla. v. Florida, 517 U. S. 44, 55 (1996) (internal quotation marks omitted). Implicit “field pre-emption” will not do.

Nor can federal power over illegal immigration be deemed exclusive because of what the Court’s opinion solicitously calls “foreign countries^] concern[s] about the status, safety, and security of their nationals in the United States,” ante, at 395. The Constitution gives all those on our shores the protections of the Bill of Rights—but just as those rights are not expanded for foreign nationals because of their countries’ views (some countries, for example, have recently discovered the death penalty to be barbaric), neither are the fundamental sovereign powers of the States abridged to accommodate foreign countries’ views. Even in its international relations, the Federal Government must live with the inconvenient fact that it is a Union of independent States, who have their own sovereign powers. This is not the first time it has found that a nuisance and a bother in the conduct of foreign policy. Four years ago, for example, the Government importuned us to interfere with thoroughly constitutional state judicial procedures in the criminal trial of foreign nationals because *424the international community, and even an opinion of the International Court of Justice, disapproved them. See Medellín v. Texas, 552 U. S. 491 (2008). We rejected that request, as we should reject the Executive’s invocation of foreign-affairs considerations here. Though it may upset foreign powers—and even when the Federal Government desperately wants to avoid upsetting foreign powers—the States have the right to protect their borders against foreign nationals, just as they have the right to execute foreign nationals for murder.

What this case comes down to, then, is whether the Arizona law conflicts with federal immigration law—whether it excludes those whom federal law would admit, or admits those whom federal law would exclude. It does not purport to do so. It applies only to aliens who neither possess a privilege to be present under federal law nor have been removed pursuant to the Federal Government’s inherent authority. I proceed to consider the challenged provisions in detail.

§2(B)

“For any lawful stop, detention or arrest made by a law enforcement official ... in the enforcement of any other law or ordinance of a county, city or town or this state where reasonable suspicion exists that the person is an alien and is unlawfully present in the United States, a reasonable attempt shall be made, when practicable, to determine the immigration status of the person, except if the determination may hinder or obstruct an investigation. Any person who is arrested shall have the person’s immigration status determined before the person is released....” S. B. 1070, § 2(B), as amended, Ariz. Rev. Stat. Ann. § 11-1051(B) (West 2012).

The Government has conceded that “even before Section 2 was enacted, state and local officers had state-law authority to inquire of DHS [the Department of Homeland Security] *425about a suspect’s unlawful status and otherwise cooperate with federal immigration officers.” Brief for United States 47 (citing App. 62, 82); see also Brief for United States 48-49. That concession, in my view, obviates the need for further inquiry. The Government’s conflict-pre-emption claim calls on us “to determine whether, under the circumstances of this particular case, [the State’s] law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.” Hines v. Davidowitz, 312 U. S. 52, 67 (1941) (emphasis added). It is impossible to make such a finding without a factual record concerning the manner in which Arizona is implementing these provisions—something the Government’s preenforcement challenge has pretermit-ted. “The fact that [a law] might operate unconstitutionally under some conceivable set of circumstances is insufficient to render it wholly invalid, since we have not recognized an ‘overbreadth’ doctrine outside the limited context of the First Amendment.” United States v. Salerno, 481 U. S. 739, 745 (1987). And on its face, § 2(B) merely tells state officials that they are authorized to do something that they were, by the Government’s concession, already authorized to do.

The Court therefore properly rejects the Government’s challenge, recognizing that, “[a]t this stage, without the benefit of a definitive interpretation from the state courts, it would be inappropriate to assume § 2(B) will be construed in a way that creates a conflict with federal law.” Ante, at 415. Before reaching that conclusion, however, the Court goes to great length to assuage fears that “state officers will be required to delay the release of some detainees for no reason other than to verify their immigration status.” Ante, at 413. Of course, any investigatory detention, including one under §2(B), may become an “unreasonable . . . seizur[e],” U. S. Const., Amdt. 4, if it lasts too long. See Illinois v. Caballes, 543 U. S. 405, 407 (2005). But that has nothing to do with this case, in which the Government claims that § 2(B) is preempted by federal immigration law, not that anyone’s Fourth *426Amendment rights have been violated. And I know of no reason why a protracted detention that does not violate the Fourth Amendment would contradict or conflict with any federal immigration law.

§6

“A peace officer, without a warrant, may arrest a person if the officer has probable cause to believe . ..

[t]he person to be arrested has committed any public offense that makes the person removable from the United States.” S. B. 1070, § 6(A)(5), Ariz. Rev. Stat. Ann. § 13-3883(A)(5) (West Supp. 2011).

This provision of S. B. 1070 expands the statutory list of offenses for which an Arizona police officer may make an arrest without a warrant. See § 13-3883. If an officer has probable cause to believe that an individual is “removable” by reason of a public offense, then a warrant is not required to make an arrest. The Government’s primary contention is that § 6 is pre-empted by federal immigration law because it allows state officials to make arrests “without regard to federal priorities.” Brief for United States 53. The Court’s opinion focuses on limits that Congress has placed on federal officials’ authority to arrest removable aliens and the possibility that state officials will make arrests “to achieve [Arizona’s] own immigration policy” and “without any input from the Federal Government.” Ante, at 408.

Of course on this preenforcement record there is no reason to assume that Arizona officials will ignore federal immigration policy (unless it be the questionable policy of not wanting to identify illegal aliens who have committed offenses that make them removable). As Arizona points out, federal law expressly provides that state officers may “cooperate with the Attorney General in the identification, apprehension, detention, or removal of aliens not lawfully present in the United States,” 8 U. S. C. § 1357(g)(10)(B); and “cooperation]” requires neither identical efforts nor prior federal ap*427proval. It is consistent with the Arizona statute, and with the “cooperative]” system that Congress has created, for state officials to arrest a removable alien, contact federal immigration authorities, and follow their lead on what to do next. And it is an assault on logic to say that identifying a removable alien and holding him for federal determination whether he should be removed “violates the principle that the removal process is entrusted to the discretion of the Federal Government,” ante, at 409. The State’s detention does not represent commencement of the removal process unless the Federal Government makes it so.

But that is not the most important point.. The most important point is that, as we have discussed, Arizona is entitled to have “its own immigration policy”—including a more rigorous enforcement policy—so long as that does not conflict with federal law. The Court says, as though the point is utterly dispositive, that “it is not a crime for a removable alien to remain present in the United States,” ante, at 407. It is not a federal crime, to be sure. But there is no reason Arizona cannot make it a state crime for a removable alien (or any illegal alien, for that matter) to remain present in Arizona.

The Court quotes § 1226(a), which provides that, “[o]n a warrant issued by the Attorney General, an alien may be arrested and detained pending a decision on whether the alien is to be removed from the United States.” Section 1357(a)(2) also provides that a federal immigration official “shall have power without warrant... to arrest any alien in the United States, if he has reason to believe that the alien so arrested is in the United States in violation of any [federal immigration] law or regulation and is likely to escape before a warrant can be obtained for his arrest.” But statutory limitations upon the actions of federal officers in enforcing the United States’ power to protect its borders do not on their face apply to the actions of state officers in enforcing the State’s power to protect its borders. There is no more reason to read these provisions as implying that state offi*428cials are subject to similar limitations than there is to read them as implying that only federal officials may arrest removable aliens. And in any event neither implication would constitute the sort of clear elimination of the States’ sovereign power that our cases demand.

The Court raises concerns about “unnecessary harassment of some aliens . . . who federal officials determine should not be removed.” Ante, at 408. But we have no license to assume, without any support in the record, that Arizona officials would use their arrest authority under §6 to harass anyone. And it makes no difference that federal officials might “determine [that some unlawfully present aliens] should not be removed,” ibid. They may well determine not to remove from the United States aliens who have no right to be here; but unless and until these aliens have been given the right to remain, Arizona is entitled to arrest them and at least bring them to federal officials’ attention, which is all that §6 necessarily entails. (In my view, the State can go further than this, and punish them for their unlawful entry and presence in Arizona.)

The Government complains that state officials might not heed “federal priorities.” Indeed they might not, particularly if those priorities include willful blindness or deliberate inattention to the presence of removable aliens in Arizona. The State’s whole complaint—the reason this law was passed and this case has arisen—is that the citizens of Arizona believe federal priorities are too lax. The State has the sovereign power to protect its borders more rigorously if it wishes, absent any valid federal prohibition. The Executive’s policy choice of lax federal enforcement does not constitute such a prohibition.

§3

“In addition to any violation of federal law, a person is guilty of willful failure to complete or carry an alien registration document if the person is in violation of 8 *429[U.S.C.] § 1304(e) or 1306(a).” S. B. 1070, §3(A), as amended, Ariz. Rev. Stat. Ann. § 13-1509(A).

It is beyond question that a State may make violation of federal law a violation of state law as well. We have held that to be so even when the interest protected is a distinctively federal interest, such as protection of the dignity of the national flag, see Halter v. Nebraska, 205 U. S. 34 (1907), or protection of the Federal Government’s ability to recruit soldiers, Gilbert v. Minnesota, 254 U. S. 325 (1920). “[T]he State is not inhibited from making the national purposes its own purposes to the extent of exerting its police power to prevent its own citizens from obstructing the accomplishment of such purposes.” Id., at 331 (internal quotation marks omitted). Much more is that so when, as here, the State is protecting its own interest, the integrity of its borders. And we have said that explicitly with regard to illegal immigration: “Despite the exclusive federal control of this Nation’s borders, we cannot conclude that the States are without any power to deter the influx of persons entering the United States against federal law, and whose numbers might have a discernible impact on traditional state concerns.” Plyler v. Doe, 457 U. S. 202, 228, n. 23 (1982).

The Court’s opinion relies upon Hines v. Davidowitz, 312 U. S. 52. Ante, at 401. But that case did not, as the Court believes, establish a “field pre-emption” that implicitly eliminates the States’ sovereign power to exclude those whom federal law excludes. It held that the States are not permitted to establish “additional or auxiliary” registration requirements for aliens. 312 U. S., at 66-67. But § 3 does not establish additional or auxiliary registration requirements. It merely makes a violation of state law the very same failure to register and failure to carry evidence of registration that are violations of federal law. Hines does not prevent the State from relying on the federal registration system as “an available aid in the enforcement of a number of statutes of the state applicable to aliens whose constitutional validity *430has not been questioned.” Id., at 75-76 (Stone, J., dissenting). One such statute is Arizona’s law forbidding illegal aliens to collect unemployment benefits, Ariz. Rev. Stat. Ann. § 23-781(B) (West 2012). To enforce that and other laws that validly turn on alien status, Arizona has, in Justice Stone’s words, an interest in knowing “the number and whereabouts of aliens within the state” and in having “a means of their identification,” 312 U. S., at 75. And it can punish the aliens’ failure to comply with the provisions of federal law that make that knowledge and identification possible.

In some areas of uniquely federal concern—e. g., fraud in a federal administrative process (Buckman Co. v. Plaintiffs’ Legal Comm., 531 U. S. 341 (2001)) or perjury in violation of a federally required oath (In re Loney, 134 U. S. 372 (1890))— this Court has held that a State has no legitimate interest in enforcing a federal scheme. But the federal alien registration system is certainly not of uniquely federal interest. States, private entities, and individuals rely on the federal registration system (including the E-Verify program) on a regular basis. Arizona’s legitimate interest in protecting (among other things) its unemployment-benefits system is an entirely adequate basis for making the violation of federal registration and carry requirements a violation of state law as well.

The Court points out, however, ante, at 402-403, that in some respects the state law exceeds the punishments prescribed by federal law: It rules out probation and pardon, which are available under federal law. The answer is that it makes no difference. Illegal immigrants who violate § 3 violate Arizona law. It is one thing to say that the Supremacy Clause prevents Arizona law from excluding those whom federal law admits. It is quite something else to say that a violation of Arizona law cannot be punished more severely than a violation of federal law. Especially where (as here) the State is defending its own sovereign interests, there is no precedent for such a limitation. The sale of illegal drugs, for example, ordinarily violates state law as well as federal *431law, and no one thinks that the state penalties cannot exceed the federal. As I have discussed, moreover, “field preemption” cannot establish a prohibition of additional state penalties in the area of immigration.

Finally, the Government also suggests that §3 poses an obstacle to the administration of federal immigration law, see Brief for United States 31-33, but “there is no conflict in terms, and no possibility of such conflict, [if] the state statute makes federal law its own,” California v. Zook, 336 U. S. 725, 735 (1949).

It holds no fear for me, as it does for the Court, that “[w]ere §3 to come into force, the State would have the power to bring criminal charges against individuals for violating a federal law even in circumstances where federal officials in charge of the comprehensive scheme determine that prosecution would frustrate federal policies.” Ante, at 402. That seems to me entirely appropriate when the State uses the federal law (as it must) as the criterion for the exercise of its own power, and the implementation of its own policies of excluding those who do not belong there. What I do fear—and what Arizona and the States that support it fear— is that “federal policies” of nonenforcement will leave the States helpless before those evil effects of illegal immigration that the Court’s opinion dutifully recites in its prologue {ante, at 397-398) but leaves unremedied in its disposition.

§5(C)

“It is unlawful for a person who is unlawfully present in the United States and who is an unauthorized alien to knowingly apply for work, solicit work in a public place or perform work as an employee or independent contractor in this state.” S. B. 1070, § 5(C), as amended, Ariz. Rev. Stat. Ann. § 13-2928(0 (West Supp. 2011).

Here, the Court rightly starts with De Canas v. Bica, 424 U. S. 351 (1976), which involved a California law providing that “ ‘[n]o employer shall knowingly employ an alien who is not entitled to lawful residence in the United States if *432such employment would have an adverse effect on lawful resident workers.’” Id., at 352 (quoting Cal. Lab. Code Ann. § 2805(a)). This Court concluded that the California law was not pre-empted, as Congress had neither occupied the field of “regulation of employment of illegal aliens” nor expressed “the clear and manifest purpose” of displacing such state regulation. 424 U. S., at 356-357 (internal quotation marks omitted). Thus, at the time De Canas was decided, §5(C) would have been indubitably lawful.

The only relevant change is that Congress has since enacted its own restrictions on employers who hire illegal aliens, 8 U. S. C. § 1324a, in legislation that also includes some civil (but no criminal) penalties on illegal aliens who accept unlawful employment. The Court concludes from this (reasonably enough) “that Congress made a deliberate choice not to impose criminal penalties on aliens who seek, or engage in, unauthorized employment,” ante, at 405. But that is not the same as a deliberate choice to prohibit the States from imposing criminal penalties. Congress’s intent with regard to exclusion of state law need not be guessed at, but is found in the law’s express pre-emption provision, which excludes “any State or local law imposing civil or criminal sanctions (other than through licensing and similar laws) upon those who employ, or recruit or refer for a fee for employment, unauthorized aliens,” §1324a(h)(2) (emphasis added). Common sense, reflected in the canon expressio unius est exclusio alterius, suggests that the specification of pre-emption for laws punishing “those who employ” implies the lack of pre-emption for other laws, including laws punishing “those who seek or accept employment.”

The Court has no credible response to this. It quotes our jurisprudence to the effect that an “express pre-emption provisio[n] does not bar the ordinary working of conflict preemption principles.” Ante, at 406 (quoting Geier v. American Honda Motor Co., 529 U. S. 861, 869 (2000) (internal quotation marks omitted)). True enough—conflict pre-emption *433principles. It then goes on to say that since “Congress decided it would be inappropriate to impose criminal penalties on aliens who seek or engage in unauthorized employment,” “[i]t follows that a state law to the contrary is an obstacle to the regulatory system Congress chose.” Ante, at 406. For “ ‘[wjhere a comprehensive federal scheme intentionally leaves a portion of the regulated field without controls, then the pre-emptive inference can be drawn.’” Ante, at 406-407 (quoting Puerto Rico Dept. of Consumer Affairs v. ISLA Petroleum Corp., 485 U. S. 495, 503 (1988)). All that is a classic description not of conflict pre-emption but oí field pre-emption, which (concededly) does not occur beyond the terms of an express pre-emption provision.

The Court concludes that § 5(C) “would interfere with the careful balance struck by Congress,” ante, at 406 (another field pre-emption notion, by the way), but that is easy to say and impossible to demonstrate. The Court relies primarily on the fact that “[proposals to make unauthorized work a criminal offense were debated and discussed during the long process of drafting [the Immigration Reform and Control Act of 1986 (IRCA)],” “[b]ut Congress rejected them.” Ante, at 405. There is no more reason to believe that this rejection was expressive of a desire that there be no sanctions on employees, than expressive of a desire that such sanctions be left to the States. To tell the truth, it was most likely expressive of what inaction ordinarily expresses: nothing at all. It is a “naive assumption that the failure of a bill to make it out of committee, or to be adopted when reported to the floor, is the same as a congressional rejection of what the bill contained.” Crosby v. National Foreign Trade Council, 530 U. S. 363, 389 (2000) (Scalia, J., concurring in judgment) (internal quotation marks and brackets omitted).

* * *

The brief for the Government in this case asserted that “the Executive Branch’s ability to exercise discretion and set *434priorities is particularly important because of the need to allocate scarce enforcement resources wisely.” Brief for United States 21. Of course there is no reason why the Federal Executive’s need to allocate its scarce enforcement resources should disable Arizona from devoting its resources to illegal immigration in Arizona that in its view the Federal Executive has given short shrift. Despite Congress’s prescription that “the immigration laws of the United States should be enforced vigorously and uniformly,” IRCA § 115, 100 Stat. 3384, Arizona asserts without contradiction and with supporting citations:

“[I]n the last decade federal enforcement efforts have focused primarily on areas in California and Texas, leaving Arizona’s border to suffer from comparative neglect. The result has been the funneling of an increasing tide of illegal border crossings into Arizona. Indeed, over the past decade, over a third of the Nation’s illegal border crossings occurred in Arizona.” Brief for Petitioners 2-3 (footnote omitted).

Must Arizona’s ability to protect its borders yield to the reality that Congress has provided inadequate funding for federal enforcement—or, even worse, to the Executive’s unwise targeting of that funding?

But leave that aside. It has become clear that federal enforcement priorities—in the sense of priorities based on the need to allocate “scarce enforcement resources”—is not the problem here. After this case was argued and while it was under consideration, the Secretary of Homeland Security announced a program exempting from immigration enforcement some 1.4 million illegal immigrants under the age of 30.4 If an individual unlawfully present in the United States

“• came to the United States under the age of sixteen;

“• has continuously resided in the United States for at least five years .. .;

*435“• is currently in school, has graduated from high school, has obtained a general education development certificate, or is an honorably discharged veteran . . . ;

“• has not been convicted of a [serious crime]; and

“• is not above the age of thirty,”5

then U. S. immigration officials have been directed to “defe[r] action” against such individual “for a period of two years, subject to renewal.”6 The husbanding of scarce enforcement resources can hardly be the justification for this, since the considerable administrative cost of conducting as many as 1.4 million background checks, and ruling on the biennial requests for dispensation that the nonenforcement program envisions, will necessarily be deducted from immigration enforcement. The President said at a news conference that the new program is “the right thing to do” in light of Congress’s failure to pass the administration’s proposed revision of the Immigration Act.7 Perhaps it is, though Arizona may not think so. But to say, as the Court does, that Arizona contradicts federal law by enforcing applications of the Immigration Act that the President declines to enforce boggles the mind.

The Court opinion’s looming specter of inutterable horror—“[i]f § 3 of the Arizona statute were valid, every State could give itself independent authority to prosecute federal registration violations,” ante, at 402—seems to me not so horrible and even less looming. But there has come to pass, and is with us today, the specter that Arizona and the States *436that support it predicted: a Federal Government that does not want to enforce the immigration laws as written, and leaves the States’ borders unprotected against immigrants whom those laws would exclude. So the issue is a stark one. Are the sovereign States at the mercy of the Federal Executive’s refusal to enforce the Nation’s immigration laws?

A good way of answering that question is to ask: Would the States conceivably have entered into the Union if the Constitution itself contained the Court’s holding? Today’s judgment surely fails that test. At the Constitutional Convention of 1787, the delegates contended with “the jealousy of the states with regard to their sovereignty.” 1 Records of the Federal Convention 19 (M. Farrand ed. 1911) (statement of Edmund Randolph). Through ratification of the fundamental charter that the Convention produced, the States ceded much of their sovereignty to the Federal Government. But much of it remained jealously guarded—as reflected in the innumerable proposals that never left Independence Hall. Now, imagine a provision—perhaps inserted right after Art. I, § 8, cl. 4, the Naturalization Clause—which included among the enumerated powers of Congress “To establish Limitations upon Immigration that will be exclusive and that will be enforced only to the extent the President deems appropriate.” The delegates to the Grand Convention would have rushed to the exits.

As is often the case, discussion of the dry legalities that are the proper object of our attention suppresses the very human realities that gave rise to the suit. Arizona bears the brunt of the country’s illegal immigration problem. Its citizens feel themselves under siege by large numbers of illegal immigrants who invade their property, strain their social services, and even place their lives in jeopardy. Federal officials have been unable to remedy the problem, and indeed have recently shown that they are unwilling to do so. Thousands of Arizona’s estimated 400,000 illegal immigrants—including not just children but men and women under 30—are *437now assured immunity from enforcement, and will be able to compete openly with Arizona citizens for employment.

Arizona has moved to protect its sovereignty—not in contradiction of federal law, but in complete compliance with it. The laws under challenge here do not extend or revise federal immigration restrictions, but merely enforce those restrictions more effectively. If securing its territory in this fashion is not within the power of Arizona, we should cease referring to it as a sovereign State. I dissent.

Justice Thomas,

concurring in part and dissenting in part.

I agree with Justice Scalia that federal immigration law does not pre-empt any of the challenged provisions of S. B. 1070. I reach that conclusion, however, for the simple reason that there is no conflict between the “ordinary meaning]” of the relevant federal laws and that of the four provisions of Arizona law at issue here. Wyeth v. Levine, 555 U. S. 555, 588 (2009) (Thomas, J., concurring in judgment) (“Pre-emption analysis should not be a freewheeling judicial inquiry into whether a state statute is in tension with federal objectives, but an inquiry into whether the ordinary meanings of state and federal law conflict” (brackets and internal quotation marks omitted)).

Section 2(B) of S. B. 1070 provides that, when Arizona law enforcement officers reasonably suspect that a person they have lawfully stopped, detained, or arrested is unlawfully present, “a reasonable attempt shall be made, when practicable, to determine the immigration status of the person” pursuant to the verification procedure established by Congress in 8 U.S.C. § 1373(c). Ariz. Rev. Stat. Ann. § 11-1051(B) (West 2012). Nothing in the text of that or any other federal statute prohibits Arizona from directing its officers to make immigration-related inquiries in these situations. To the contrary, federal law expressly states that “no State or local government entity may be prohibited, or in any way *438restricted, from sending to or receiving from” federal officials “information regarding the immigration status” of an alien. 8 U. S. C. § 1644. And, federal law imposes an affirmative obligation on federal officials to respond to a State’s immigration-related inquiries. § 1373(c).

Section 6 of S. B. 1070 authorizes Arizona law enforcement officers to make warrantless arrests when there is probable cause to believe that an arrestee has committed a public offense that renders him removable under federal immigration law. States, as sovereigns, have inherent authority to conduct arrests for violations of federal law, unless and until Congress removes that authority. See United States v. Di Re, 332 U. S. 581, 589 (1948) (holding that state law determines the validity of a warrantless arrest for a violation of federal law “in [the] absence of an applicable federal statute”). Here, no federal statute purports to withdraw that authority. As Justice Scalia notes, ante, at 426 (opinion concurring in part and dissenting in part), federal law does limit the authority of federal officials to arrest removable aliens, but those statutes do not apply to state officers. And, federal law expressly recognizes that state officers may “cooperate with the Attorney General” in the “apprehension” and “detention” of “aliens not lawfully present in the United States.” § 1357(g)(10)(B). Nothing in that statute indicates that such cooperation requires a prior “request, approval, or other instruction from the Federal Government.” Ante, at 410 (majority opinion).

Section 3 of S. B. 1070 makes it a crime under Arizona law for an unlawfully present alien to willfully fail to complete or carry an alien registration document in violation of 8 U. S. C. §§ 1304(e) and 1306(a). Section 3 simply incorporates federal registration standards. Unlike the Court, I would not hold that Congress pre-empted the field of enforcing those standards. “[O]ur recent cases have frequently rejected field pre-emption in the absence of statutory language expressly requiring it.” Camps Newfound/Owatonna, Inc. *439v. Town of Harrison, 520 U. S. 564, 617 (1997) (Thomas, J., dissenting); see, e. g., New York State Dept. of Social Servs. v. Dublino, 413 U. S. 405, 415 (1973). Here, nothing in the text of the relevant federal statutes indicates that Congress intended enforcement of its registration requirements to be exclusively the province of the Federal Government. That Congress created a “full set of standards governing alien registration,” ante, at 401 (majority opinion), merely indicates that it intended the scheme to be capable of working on its own, not that it wanted to preclude the States from enforcing the federal standards. Hines v. Davidowitz, 312 U. S. 52 (1941), is not to the contrary. As Justice Scalia explains, ante, at 429, Hines at most holds that federal law pre-empts the States from creating additional registration requirements. But here, Arizona is merely seeking to enforce the very registration requirements that Congress created.

Section 5(C) of S. B. 1070 prohibits unlawfully present aliens from knowingly applying for, soliciting, or performing work in Arizona. Section 5(C) operates only on individuals whom Congress has already declared ineligible to work in the United States. Nothing in the text of the federal immigration laws prohibits States from imposing their own criminal penalties on such individuals. Federal law expressly pre-empts States from “imposing civil or criminal sanctions (other than through licensing and similar laws) upon those who employ, or recruit or refer for a fee for employment, unauthorized aliens.” 8 U. S. C. § 1324a(h)(2) (emphasis added). But it leaves States free to impose criminal sanctions on the employees themselves.

Despite the lack of any conflict between the ordinary meaning of the Arizona law and that of the federal laws at issue here, the Court holds that various provisions of the Arizona law are pre-empted because they “stan[d] as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.” Hines, supra, at 67. *440I have explained that the “purposes and objectives” theory of implied pre-emption is inconsistent with the Constitution because it invites courts to engage in freewheeling speculation about congressional purpose that roams well beyond statutory text. See Wyeth, 555 U. S., at 604 (opinion concurring in judgment); see also Williamson v. Mazda Motor of America, Inc., 562 U. S. 323, 340-341 (2011) (opinion concurring in judgment); Haywood v. Drown, 556 U. S. 729, 767 (2009) (dissenting opinion). Under the Supremacy Clause, pre-emptive effect is to be given to congressionally enacted laws, not to judicially divined legislative purposes. See Wyeth, supra, at 604 (Thomas, J., concurring in judgment). Thus, even assuming the existence of some tension between Arizona’s law and the supposed “purposes and objectives” of Congress, I would not hold that any of the provisions of the Arizona law at issue here are pre-empted on that basis.

Justice Alito,

concurring in part and dissenting in part.

This case concerns four provisions of Arizona’s Support Our Law Enforcement and Safe Neighborhoods Act, S. B. 1070. Section 2(B) requires Arizona law enforcement officers to make a “reasonable attempt,” “when practicable,” to ascertain the immigration status of any person who an officer lawfully stops, detains, or arrests “where reasonable suspicion exists that the person is an alien and is unlawfully present in the United States.” Ariz. Rev. Stat. Ann. §11-1051(B) (West 2012). Section 3 provides that an alien who willfully fails “to complete or carry an alien registration document” in violation of 8 U. S. C. § 1304(e) or § 1306(a) is guilty of a misdemeanor. Ariz. Rev. Stat. Ann. § 13-1509(A) (West Supp. 2011). Section 5(C) makes it a misdemeanor for an unauthorized alien who is unlawfully present in the United States “to knowingly apply for work, solicit work in a public place or perform work as an employee or independent contractor.” Ariz. Rev. Stat. Ann. §13-2928(C). And §6 au*441thorizes Arizona law enforcement officers to arrest without a warrant any person who an officer has probable cause to believe “has committed any public offense that makes the person removable from the United States.” Ariz. Rev. Stat. Ann. § 13-3883(A)(5).

I agree with the Court that § 2(B) is not pre-empted. That provision does not authorize or require Arizona law enforcement officers to do anything they are not already allowed to do under existing federal law. The United States’ argument that § 2(B) is pre-empted, not by any federal statute or regulation, but simply by the Executive’s current enforcement policy is an astounding assertion of federal executive power that the Court rightly rejects.

I also agree with the Court that § 3 is pre-empted by virtue of our decision in Hines v. Davidowitz, 312 U. S. 52 (1941). Our conclusion in that ease that Congress had enacted an “all-embracing system” of alien registration and that States cannot “enforce additional or auxiliary regulations,” id., at 66-67, 74, forecloses Arizona’s attempt here to impose additional, state-law penalties for violations of the federal registration scheme.

While I agree with the Court on §§2(B) and 3,1 part ways on § § 5(C) and 6. The Court’s holding on §5(C) is inconsistent with De Canas v. Bica, 424 U. S. 351 (1976), which held that employment regulation, even of aliens unlawfully present in the country, is an area of traditional state concern. Because state police powers are implicated here, our precedents require us to presume that federal law does not displace state law unless Congress’ intent to do so is clear and manifest. I do not believe Congress has spoken with the requisite clarity to justify invalidation of §5(C). Nor do I believe that §6 is invalid. Like §2(B), §6 adds virtually nothing to the authority that Arizona law enforcement officers already exercise. And whatever little authority they have gained is consistent with federal law.

*442 Section 2(B)

A

Although §2(B) of the Arizona law has occasioned much controversy, it adds nothing to the authority that Arizona law enforcement officers, like officers in all other States, already possess under federal law. For that reason, I agree with the Court that § 2(B) is not pre-empted.

Section 2(B) quite clearly does not expand the authority of Arizona officers to make stops or arrests. It is triggered only when a “lawful stop, detention or arrest [is] made . . . in the enforcement of any other [state or local] law or ordinance .” Ariz. Rev. Stat. Ann. §11-1051(B) (emphasis added). Section 2(B) thus comes into play only when an officer has reasonable suspicion or probable cause to believe that a person has committed a nonimmigration offense. Arizona officers plainly possessed this authority before §2(B) took effect.

Section 2(B) also does not expand the authority of Arizona officers to inquire about the immigration status of persons who are lawfully detained. When a person is stopped or arrested and “reasonable suspicion exists that the person is an alien and is unlawfully present in the United States,” §2(B) instructs Arizona officers to make a “reasonable attempt,” “when practicable,” to ascertain that person’s immigration status. Ariz. Rev. Stat. Ann. § 11-1051(B). Even before the Arizona Legislature enacted §2(B), federal law permitted state and local officers to make such inquiries. In 8 U. S. C. § 1357(g)(10)(A), Congress has made clear that state and local governments need not enter into formal agreements with the Federal Government in order “to communicate with the [Federal Government] regarding the immigration status of any individual.” In addition, Congress has mandated that neither the Federal Government nor any state or local government may “prohibit, or in any way restrict, any government entity or official from sending *443to, or receiving from, [the Federal Government] information regarding the citizenship or immigration status, lawful or unlawful, of any individual.” § 1373(a); see also §1644 (providing that “no State or local government entity may be prohibited, or in any way restricted, from sending to or receiving from [the Federal Government] information regarding the immigration status, lawful or unlawful, of an alien in the United States”). And while these provisions preserve the authority of state and local officers to seek immigration-status information from the Federal Government, another federal statute, § 1373(c), requires that the Federal Government respond to any such inquiries “by providing the requested verification or status information.” It comes as no surprise, therefore, that many States and localities permit their law enforcement officers to make the kinds of inquiries that §2(B) prescribes. See App. 294-298 (reporting that officers in 59 surveyed state and local jurisdictions “generally” ask arrestees about their immigration status while 34 do not and that officers in 78 jurisdictions “generally” inform Immigration and Customs Enforcement (ICE) when they believe an arrestee to be an undocumented alien while only 17 do not). Congress has invited state and local governments to make immigration-related inquiries and has even obligated the Federal Government to respond. Through §2(B), Arizona has taken Congress up on that invitation.

The United States does not deny that officers may, at their own discretion, inquire about the immigration status of persons whom they lawfully detain. Instead, the United States argues that § 2(B) is pre-empted because it impedes federal-state cooperation by mandating that officers verify the immigration status of every detained person if there is reason to believe that the person is unlawfully present in the country. The United States claims that §2(B)’s mandate runs contrary to federal law in that it “precludes officers from taking [the Federal Government’s] priorities and discretion *444into account.” Brief for United States 50. “[B]y interposing a mandatory state law between state and local officers and their federal counterparts,” writes the United States, §2(B) “stands as an obstacle to the accomplishment of the federal requirement of cooperation and the full effectuation of the enforcement judgment and discretion Congress has vested in the Executive Branch.” Ibid, (internal quotation marks and citation omitted).

The underlying premise of the United States’ argument seems to be that state and local officers, when left to their own devices, generally take federal enforcement priorities into account. But there is no reason to think that this premise is true. And even if it were, it would not follow that § 2(B)’s blanket mandate is at odds with federal law. Nothing in the relevant federal statutes requires state and local officers to consider the Federal Government’s priorities before requesting verification of a person’s immigration status. Neither 8 U. S. C. § 1357(g)(10) nor § 1373(a) conditions the right of state and local officers to communicate with the Federal Government on their first taking account of its priorities. Nor does § 1373(c) condition the Federal Government’s obligation to answer requests for information on the sensitivity of state and local officers to its enforcement discretion. In fact, § 1373(c) dictates that the Federal Government “shall respond” to any inquiry seeking verification of immigration status, and that command applies whether or not the requesting officer has bothered to consider federal priorities. Because no federal statute requires such consideration, § 2(B) does not conflict with federal law.

In any event, it is hard to see how state and local officers could proceed in conformity with the Federal Government’s enforcement priorities without making an inquiry into a suspected alien’s immigration status. For example, one of the Federal Government’s highest priorities is the apprehension and removal of aliens who have failed to comply with a final order of removal. See App. 108. How can an officer iden*445tify those persons without first inquiring about their status? At bottom, the discretion that ultimately matters is not whether to verify a person’s immigration status but whether to act once the person’s status is known. For that reason, §2(B)’s verification requirement is not contrary to federal law because the Federal Government retains the discretion that matters most—that is, the discretion to enforce the law in particular cases. If an Arizona officer contacts the Federal Government to verify a person’s immigration status and federal records reveal that the person is in the country unlawfully, the Federal Government decides, presumably based on its enforcement priorities, whether to have the person released or transferred to federal custody. Enforcement discretion thus lies with the Federal Government, not with Arizona. Nothing in § 2(B) suggests otherwise.

The United States’ attack on § 2(B) is quite remarkable. The United States suggests that a state law may be preempted, not because it conflicts with a federal statute or regulation, but because it is inconsistent with a federal agency’s current enforcement priorities. Those priorities, however, are not law. They are nothing more than agency policy. I am aware of no decision of this Court recognizing that mere policy can have pre-emptive force. Cf. Barclays Bank PLC v. Franchise Tax Bd. of Cal., 512 U. S. 298, 380 (1994) (holding that “Executive Branch communications that express federal policy but lack the force of law cannot render unconstitutional” an “otherwise valid, congressionally condoned” state law). If § 2(B) were pre-empted at the present time because it is out of sync with the Federal Government’s current priorities, would it be unpre-empted at some time in the future if the agency’s priorities changed?

Like most law enforcement agencies, ICE does not set out inflexible rules for its officers to follow. To the contrary, it provides a list of factors to guide its officers’ enforcement discretion on a case-by-case basis. See Memorandum from John Morton, Director, ICE, to All Field Office Directors *446et al., Exercising Prosecutorial Discretion Consistent With the Civil Immigration Enforcement Priorities of the Agency for the Apprehension, Detention, and Removal of Aliens 4 (June 17, 2011) (“This list is not exhaustive and no one factor is determinative. ICE officers, agents, and attorneys should always consider prosecutorial discretion on a case-by-case basis. The decisions should be based on the totality of the circumstances, with the goal of conforming to ICE’s enforcement priorities”). Among those factors is “the agency’s civil immigration enforcement priorities,” ibid., which change from administration to administration. If accepted, the United States’ pre-emption argument would give the Executive unprecedented power to invalidate state laws that do not meet with its approval, even if the state laws are otherwise consistent with federal statutes and duly promulgated regulations. This argument, to say the least, is fundamentally at odds with our federal system.

B

It has been suggested that § 2(B) will cause some persons who are lawfully stopped to be detained in violation of their constitutional rights while a prolonged investigation of their immigration status is undertaken. But nothing on the face of the law suggests that it will be enforced in a way that violates the Fourth Amendment or any other provision of the Constitution. The law instructs officers to make a “reasonable attempt” to investigate immigration status, and this language is best understood as incorporating the Fourth Amendment’s standard of reasonableness. Indeed, the Arizona Legislature has directed that §2(B) “shall be implemented in a manner consistent with federal laws . . . protecting the civil rights of all persons and respecting the privileges and immunities of United States citizens.” Ariz. Rev. Stat. Ann. § 11-1051(L).

In the situations that seem most likely to occur, enforcement of § 2(B) will present familiar Fourth Amendment ques*447tions. To take a common situation, suppose that a car is stopped for speeding, a nonimmigration offense. (Recall that §2(B) comes into play only where a stop or arrest is made for a nonimmigration offense.) Suppose also that the officer who makes the stop subsequently acquires reasonable suspicion to believe that the driver entered the country illegally, which is a federal crime. See 8 U. S. C. § 1325(a).

It is well established that state and local officers generally have authority to make stops and arrests for violations of federal criminal laws. See, e. g., Miller v. United States, 357 U. S. 301, 305 (1958); United States v. Di Re, 332 U. S. 581, 589 (1948). I see no reason why this principle should not apply to immigration crimes as well. Lower courts have so held. See, e. g., Estrada v. Rhode Island, 594 F. 3d 56, 65 (CA1 2010) (upholding the lawfulness of a detention because the officer had an objectively reasonable belief that the ar-restees “had committed immigration violations”); United States v. Vasquez-Alvarez, 176 F. 3d 1294, 1296 (CA10 1999) (noting that “state law-enforcement officers have the general authority to investigate and make arrests for violations of federal immigration laws”); Gonzales v. Peoria, 722 F. 2d 468, 475 (CA9 1983), overruled on other grounds, Hodgers-Durgin v. de la Vina, 199 F. 3d 1037 (1999) (en banc) (holding that “federal law does not preclude local enforcement of the criminal provisions” of federal immigration law). And the United States, consistent with the position long taken by the Office of Legal Counsel (OLC) in the Department of Justice, does not contend otherwise. See Brief for United States 55, n. 33; see also Memorandum from OLC to the Attorney General (Apr. 3, 2002), App. 268-273; Assistance by State and Local Police in Apprehending Illegal Aliens, 20 Op. Off. Legal Counsel 26 (1996).

More importantly, no federal statute casts doubt on this authority. To be sure, there are a handful of statutes that purport to authorize state and local officers to make immigration-related arrests in certain situations. See, e. g., *4488 U. S. C. § 1103(a)(10) (providing for the extension of “any” immigration enforcement authority to state and local officers in the event of an “actual or imminent mass influx of aliens arriving off the coast”); § 1252e(a) (providing authority to arrest criminal aliens who had illegally reentered the country but only after consultation with the Federal Government); § 1324(c) (providing authority to make arrests for transporting and harboring certain aliens). But a grant of federal arrest authority in some cases does not manifest a clear congressional intent to displace the States’ police powers in all other cases. Without more, such an inference is too weak to overcome our presumption against pre-emption where traditional state police powers are at stake. Accordingly, in our hypothetical case, the Arizona officer may arrest the driver for violating § 1325(a) if the officer has probable cause. And if the officer has reasonable suspicion, the officer may detain the driver, to the extent permitted by the Fourth Amendment, while the question of illegal entry is investigated.

We have held that a detention based on reasonable suspicion that the detainee committed a particular crime “can become unlawful if it is prolonged beyond the time reasonably required to complete that mission.” Illinois v. Caballes, 543 U. S. 405, 407 (2005). But if during the course of a stop an officer acquires suspicion that a detainee committed a different crime, the detention may be extended for a reasonable time to verify or dispel that suspicion. Cf. Muehler v. Mena, 544 U. S. 93, 101 (2005) (holding that “no additional Fourth Amendment justification” was required because any questioning concerning immigration status did not prolong the detention). In our hypothetical case, therefore, if the officer, after initially stopping the car for speeding, has a reasonable suspicion that the driver entered the country illegally, the officer may investigate for evidence of illegal entry. But the length and nature of this investigation must remain within the limits set out in our Fourth Amendment cases. *449An investigative stop, if prolonged, can become an arrest and thus require probable cause. See Caballes, supra, at 407. Similarly, if a person is moved from the site of the stop, probable cause will likely be required. See Hayes v. Florida, 470 U. S. 811, 816 (1985) (holding that the line between detention and arrest is crossed “when the police, without probable cause or a warrant, forcibly remove a person from his home or other place in which he is entitled to be and transport him to the police station, where he is detained, although briefly, for investigative purposes”).

If properly implemented, § 2(B) should not lead to federal constitutional violations, but there is no denying that enforcement of § 2(B) will multiply the occasions on which sensitive Fourth Amendment issues will crop up. These civil-liberty concerns, I take it, are at the heart of most objections to § 2(B). Close and difficult questions will inevitably arise as to whether an officer had reasonable suspicion to believe that a person who is stopped for some other reason entered the country illegally, and there is a risk that citizens, lawful permanent residents, and others who are lawfully present in the country will be detained. To mitigate this risk, Arizona could issue guidance to officers detailing the circumstances that typically give rise to reasonable suspicion of unlawful presence. And in the spirit of the federal-state cooperation that the United States champions, the Federal Government could share its own guidelines. Arizona could also provide officers with a nonexclusive list containing forms of identification sufficient under § 2(B) to dispel any suspicion of unlawful presence. If Arizona accepts licenses from most States as proof of legal status, the problem of roadside detentions will be greatly mitigated.1

*450 Section 8

I agree that §3 is pre-empted because, like the Court, I read the opinion in Hines to require that result. Although there is some ambiguity in Hines, the Court largely spoke in the language of field pre-emption. The Court explained that where Congress “has enacted a complete scheme of regulation and has therein provided a standard for the registration of aliens, states cannot, inconsistently with the purpose of Congress, conflict or interfere with, curtail or complement, the federal law, or enforce additional or auxiliary regulations.” 312 U. S., at 66-67. In finding the Pennsylvania alien-registration law pre-empted, the Court observed that Congress had “provided a standard for alien registration in a single integrated and all-embracing system” and that its intent was “to protect the personal liberties of law-abiding aliens through one uniform national registration system.” Id., at 74. If we credit our holding in Hines that Congress has enacted “a single integrated and all-embracing system” of alien registration and that States cannot “complement” that system or “enforce additional or auxiliary regulations,” id., at 66-67, 74, then Arizona’s attempt to impose additional, state-law penalties for violations of federal registration requirements must be invalidated.

Section 5(C)

While I agree that § 3 is pre-empted, I disagree with the Court’s decision to strike down § 5(C). I do so in large measure because the Court fails to give the same solicitude to our decision in De Canas, 424 U. S. 351, as it is willing to give our decision in Hines. In De Canas, the Court upheld against a pre-emption challenge a state law imposing fines on employers that hired aliens who were unlawfully present in the *451United States. The Court explained that the mere fact that “aliens are the subject of a state statute does not render it a regulation of immigration.” 424 U. S., at 355. The Court emphasized instead that “States possess broad authority under their police powers to regulate the employment relationship to protect workers within the State.” Id., at 356. In light of that broad authority, the Court declared that “[o]nly a demonstration that complete ouster of state power .. . was The clear and manifest purpose of Congress’ would justify” the conclusion that “state regulation designed to protect vital state interests must give way to paramount federal legislation.” Id., at 357 (some internal quotation marks omitted); see also Bates v. Dow Agrosciences LLC, 544 U. S. 431, 449 (2005) (“In areas of traditional state regulation, [the Court] assume[s] that a federal statute has not supplanted state law unless Congress has made such an intention ‘clear and manifest’” (some internal quotation marks omitted)).

The Court now tells us that times have changed. Since De Canas, Congress has enacted “a comprehensive framework for combating the employment of illegal aliens,” and even though aliens who seek or obtain unauthorized work are not subject to criminal sanctions, they can suffer civil penalties. Ante, at 404 (internal quotation marks omitted). Undoubtedly, federal regulation in this area is more pervasive today. But our task remains unchanged: to determine whether the federal scheme discloses a clear and manifest congressional intent to displace state law.

The Court gives short shrift to our presumption against pre-emption. Having no express statement of congressional intent to support its analysis, the Court infers from stale legislative history and from the comprehensiveness of the federal scheme that “Congress made a deliberate choice not to impose criminal penalties on aliens who seek, or engage in, unauthorized employment.” Ante, at 405. Because §5(C) imposes such penalties, the Court concludes that it stands *452as an obstacle to the method of enforcement chosen by Congress. Ante, at 406-407.

The one thing that is clear from the federal scheme is that Congress chose not to impose federal criminal penalties on aliens who seek or obtain unauthorized work. But that does not mean that Congress also chose to pre-empt state criminal penalties. The inference is plausible, but far from necessary. As we have said before, the “decision not to adopt a regulation” is not “the functional equivalent of a regulation prohibiting all States and their political subdivisions from adopting such a regulation.” Sprietsma v. Mercury Marine, 537 U. S. 51, 65 (2002). With any statutory scheme, Congress chooses to do some things and not others. If that alone were enough to demonstrate pre-emptive intent, there would be little left over for the States to regulate, especially now that federal authority reaches so far and wide. States would occupy tiny islands in a sea of federal power. This explains why state laws implicating traditional state powers are not pre-empted unless there is a “clear and manifest” congressional intention to do so.

Not only is there little evidence that Congress intended to pre-empt state laws like § 5(C), there is some evidence that Congress intended the opposite result. In making it unlawful for employers to hire unauthorized aliens, see 8 U. S. C. § 1324a(a), Congress made it clear that “any State or local law imposing civil or criminal sanctions (other than through licensing and similar laws)” upon employers was pre-empted, § 1324a(h)(2). Noticeably absent is any similar directive pre-empting state or local laws targeting aliens who seek or obtain unauthorized employment. Given that Congress expressly pre-empted certain state and local laws pertaining to employers but remained silent about laws pertaining to employees, one could infer that Congress intended to preserve state and local authority to regulate the employee side of the equation. At the very least, it raises serious *453doubts about whether Congress intended to pre-empt such authority.

The Court dismisses any inferences that might be drawn from the express pre-emption provision. See ante, at 406. But even though the existence of that provision “does not bar the ordinary working of conflict pre-emption principles” or impose a “ ‘special burden’ ” against pre-emption, Geier v. American Honda Motor Co., 529 U. S. 861, 869-870 (2000), it is still probative of congressional intent. And it is the intent of Congress that is the “ultimate touchstone.” Retail Clerks v. Schermerhorn, 375 U. S. 96, 103 (1963).

The Court infers from Congress’ decision not to impose federal criminal penalties that Congress intended to preempt state criminal penalties. But given that the express pre-emption provision covers only state and local laws regulating employers, one could just as well infer that Congress did not intend to pre-empt state or local laws aimed at alien employees who unlawfully seek or obtain work. Surely Congress’ decision not to extend its express pre-emption provision to state or local laws like § 5(C) is more probative of its intent on the subject of pre-emption than its decision not to impose federal criminal penalties for unauthorized work. In any event, the point I wish to emphasize is that inferences can be drawn either way. There are no necessary inferences that point decisively for or against pre-emption. Therefore, if we take seriously that state employment regulation is a traditional state concern and can be pre-empted only on a showing of “clear and manifest” congressional intent as required by De Canas, then § 5(C) must survive. “Our precedents establish that a high threshold must be met if a state law is to be pre-empted for conflicting with the purposes of a federal Act.” Chamber of Commerce of United States of America v. Whiting, 563 U. S. 582, 607 (2011) (plurality opinion) (internal quotation marks omitted). I do not believe the United States has surmounted that barrier here.

*454 Section 6

I also disagree with the Court’s decision that §6 is preempted. This provision adds little to the authority that Arizona officers already possess, and whatever additional authority it confers is consistent with federal law. Section 6 amended an Arizona statute that authorizes warrantless arrests. See Ariz. Rev. Stat. Ann. §13-3883 (West 2010). Before § 6 was added, that statute already permitted arrests without a warrant for felonies, misdemeanors committed in the arresting officer’s presence, petty offenses, and certain traffic-related criminal violations. See §§ 13-3883(A)(l)-(4). Largely duplicating the authority already conferred by these prior subsections, §6 added a new subsection, §13-3883(A)(5) (West Supp. 2011), that authorizes officers to make warrantless arrests on probable cause that the arrestee has committed a “public offense” for which the arrestee is removable from the United States. A “public offense” is defined as conduct that is punishable by imprisonment or a fine according to the law of the State where the conduct occurred and that would be punishable under Arizona law had the conduct occurred in Arizona. See § 13-105(27).

In what way, if any, does § 6 enlarge the arrest authority of Arizona officers? It has been suggested that §6 confers new authority in the following three circumstances: (1) where the arrestee committed but has not been charged with committing an offense in another State; (2) where the officer has probable cause to believe the arrestee committed an offense for which he was previously arrested but not prosecuted; and (3) where the arrestee committed but has already served the sentence for a removable offense. 641 F. 3d 339, 361 (CA9 2011). These are exceedingly narrow categories, involving circumstances that will rarely arise. But such cases are possible, and therefore we must decide whether there are circumstances under which federal law precludes a state officer from making an arrest based on prohable cause that the arrestee committed a removable offense.

*455A

The idea that state and local officers may carry out arrests in the service of federal law is not unprecedented. As previously noted, our cases establish that state and local officers may make warrantless arrests for violations of federal law and that in the “absence of an applicable federal statute the law of the state where an arrest without warrant takes place determines its validity.” Di Re, 332 U. S., at 589; see also Miller, 357 U. S., at 305 (stating that, where a state officer makes an arrest based on federal law, “the lawfulness of the arrest without warrant is to be determined by reference to state law”). Therefore, given the premise, which I understand both the United States and the Court to accept, that state and local officers do have inherent authority to make arrests in aid of federal law, we must ask whether Congress has done anything to curtail or pre-empt that authority in this particular case.

Neither the United States nor the Court goes so far as to say that state and local officers have no power to arrest criminal aliens based on their removability. To do so would fly in the face of 8 U. S. C. § 1357(g)(10). Under §§ 1357(g)(l)-(9), the Federal Government may enter into formal agreements with States and municipalities under which their officers may perform certain duties of a federal immigration officer. But §1357(g)(10)(B) makes clear that States and municipalities need not enter into those agreements “otherwise to cooperate ... in the identification, apprehension, detention, or removal of aliens not lawfully present in the United States.” It goes without saying that state and local officers could not provide meaningful cooperation in the apprehension, detention, and ultimate removal of criminal aliens without some power to make arrests.

Although §1357(g)(10) contemplates state and local authority to apprehend criminal aliens for the purpose of removal, the Court rejects out of hand any possibility that officers could exercise that authority without federal direction. *456Despite acknowledging that there is “ambiguity as to what constitutes cooperation,” the Court says that “no coherent understanding of the term would incorporate the unilateral decision of state officers to arrest an alien for being removable absent any request, approval, or other instruction from the Federal Government.” Ante, at 410. The Court adopts an unnecessarily stunted view of cooperation. No one would say that a state or local officer has failed to cooperate by making an on-the-spot arrest to enforce federal law. Unsolicited aid is not necessarily uncooperative.

To be sure, were an officer to persist in making an arrest that the officer knows is unwanted, such conduct would not count as cooperation. But nothing in the relevant federal statutes suggests that Congress does not want aliens who have committed removable offenses to be arrested.2 To the contrary, § 1226(c)(1) commands that the Executive “shall take into custody any alien” who is deportable for having committed a specified offense. And § 1226(c)(2) substantially limits the circumstances under which the Executive has discretion to release aliens held in custody under paragraph (1). So if an officer arrests an alien who is removable for having committed one of the crimes listed in § 1226(c)(1), the Federal Government is obligated to take the alien into custody.

That Congress generally requires the Executive to take custody of criminal aliens casts considerable doubt on the Court’s concern that § 6 is an obstacle to the Federal Government’s exercise of discretion. The Court claims that the authority conferred by §6 “could be exercised without any input from the Federal Government about whether an arrest is warranted in a particular case” and that this “would allow the State to achieve its own immigration policy,” resulting in the “unnecessary harassment of some aliens .. . who federal officials determine should not be removed.” Ante, at 408. But § 1226(c)(1) belies the Court’s fear. In many, if not most, *457cases involving aliens who are removable for having committed criminal offenses, Congress has left the Executive no discretion but to take the alien into custody. State and local officers do not frustrate the removal process by arresting criminal aliens. The Executive retains complete discretion over whether those aliens are ultimately removed. And once the Federal Government makes a determination that a particular criminal alien will not be removed, then Arizona officers are presumably no longer authorized under § 6 to arrest the alien.

To be sure, not all offenses for which officers have authority to arrest under § 6 are covered by § 1226(c)(1). As for aliens who have committed those offenses, Congress has given the Executive discretion under § 1226(a) over whether to arrest and detain them pending a decision on removal. But the mere fact that the Executive has enforcement discretion cannot mean that the exercise of state police powers in support of federal law is automatically pre-empted. If that were true, then state and local officers could never make arrests to enforce any federal statute because the Executive always has at least some general discretion over the enforcement of federal law as a practical matter. But even assuming that the express statutory grant of discretion in § 1226(a) somehow indicates a congressional desire to pre-empt unilateral state and local authority to arrest criminal aliens covered by that provision, § 6 is not pre-empted on its face given its substantial overlap with § 1226(c)(1).

It bears emphasizing that §6 does not mandate the warrantless apprehension of all aliens who have committed crimes for which they are removable. Instead, it only grants state and local officers permission to make such arrests. The trouble with this premature, facial challenge is that it affords Arizona no opportunity to implement its law in a way that would avoid any potential conflicts with federal law. For example, Arizona could promulgate guidelines or regulations limiting the arrest authority conferred by § 6 to *458the crimes specified in § 1226(c)(1). And to the extent § 1226(c)(1) is unclear about which exact crimes are covered,3 Arizona could go even further and identify specific crimes for which there is no doubt an alien would be removable. The point is that there are plenty of permissible applications of § 6, and the Court should not invalidate the statute at this point without at least some indication that Arizona has implemented it in a manner at odds with Congress' clear and manifest intent. We have said that a facial challenge to a statute is “the most difficult challenge to mount successfully” because “the challenger must establish that no set of circumstances exists under which the [statute] would be valid.” United States v. Salerno, 481 U. S. 739, 745 (1987); see also Anderson v. Edwards, 514 U. S. 143, 155, n. 6 (1995) (applying the Salerno standard in a pre-emption case). As to § 6, I do not believe the United States has carried that heavy burden.

B

Finally, the Court tells us that §6 conflicts with federal law because it provides state and local officers with “even greater authority to arrest aliens on the basis of possible removability than Congress has given to trained federal immigration officers.” Ante, at 408. The Court points to 8 U. S. C. § 1357(a)(2), which empowers “authorized” officers and employees of ICE to make arrests without a federal warrant if “the alien so arrested is in the United States in violation of any [immigration] law or regulation and is likely to escape before a warrant can be obtained for his arrest.” Because §6 would allow Arizona officers to make arrests “regardless of whether a federal warrant has issued or the alien is likely to escape,” ante, at 408, the Court concludes that § 6 is an obstacle to the accomplishment of Congress’ objectives. *459But § 6 is an obstacle only to the extent it conflicts with Congress’ clear and manifest intent to preclude state and local officers from making arrests except where a federal warrant has issued or the arrestee is likely to escape. By granting warrantless arrest authority tó federal officers, Congress has not manifested an unmistakable intent to strip state and local officers of their warrantless arrest authority under state law.

Likewise, limitations on federal arrest authority do not mean that the arrest authority of state and local officers must be similarly limited. Our opinion in Miller, 357 U. S. 301, is. instructive. In that case, a District of Columbia officer, accompanied by a federal officer, made an arrest based on a suspected federal narcotics offense. Id., at 303-304. The federal officer did not have statutory authorization to arrest without a warrant, but the local officer did. Id., at 305. We held that District of Columbia law dictated the lawfulness of the arrest. Id., at 305-306. Where a state or local officer makes a warrantless arrest to enforce federal law, we said that “the lawfulness of the arrest without warrant is to be determined by reference to state law.” Id., at 305. Under §6, an Arizona officer may be authorized to make an arrest that a federal officer may not be authorized to make under § 1357(a)(2). As Miller makes clear, that fact alone does not render arrests by state or local officers pursuant to §6 unlawful. Nor does it manifest a clear congressional intent to displace the exercise of state police powers that are brought to bear in aid of federal law.

Justice Rehnquist

delivered the opinion of the Court.

A municipal ordinance of the city of Camden, New Jersey, requires that at least 40% of the employees of contractors and subcontractors working on city construction projects be Camden residents. Appellant, the United Building and Construction Trades Council of Camden County and Vicinity (Council), challenges that ordinance as a violation of the Privileges and Immunities Clause, Art. IV, § 2, cl. 1, of the United States Constitution.1 The Supreme Court of New Jersey rejected appellant’s privileges and immunities attack on the ground that the ordinance discriminates on the basis of municipal, not state, residency. The court “decline[d] to apply the Privileges and Immunities Clause in the context of a municipal ordinance that has identical effects upon out-of-state citizens and New Jersey citizens not residing in the locality.” 88 N. J. 317, 342, 443 A. 2d 148, 160 (1982). We conclude that the challenged ordinance is properly subject to the strictures of the Clause. We therefore reverse the judgment of the Supreme Court of New Jersey and remand the case for a determination of the validity of the ordinance under the appropriate constitutional standard.

On August 28, 1980, the Camden City Council, acting pursuant to a statewide affirmative-action program,2 adopted an *211ordinance setting minority hiring “goals” on all public works contracts. Ordinance MC 1650, App. to Juris. Statement A36. The ordinance also created a hiring preference for Camden residents, with a separate 1-year residency requirement triggering eligibility for that preference. Ordinance MC 1650 § 1(5), App. to Juris. Statement A38. As subsequently amended, the ordinance requires that on all construction projects funded by the city:3

“The developer/contractor, in hiring for jobs, shall make every effort to employ persons residing within the City of Camden but, in no event, shall less than forty percent (40%) of the entire labor force be residents of the City of Camden.” Ordinance MC 1653 §C(IV)(b), App. to Juris. Statement A56.

*212The contractor is also obliged to ensure that any subcontractors working on such projects adhere to the same requirement. Ordinance MC 1650 § VIII, App. to Juris. Statement A46.

The amended ordinance was submitted for approval to the Chief Affirmative Action Officer of the New Jersey Treasury Department in November 1980. Following brief administrative proceedings, the ordinance was designated as a state-approved affirmative-action construction program. Appellant, an association of labor organizations representing private employees in the building and construction trades in various New Jersey counties,4 filed a notice of appeal with the Appellate Division of the New Jersey Superior Court challenging the final determination of the Treasury Department in approving the Camden plan. The New Jersey Supreme Court certified the appeal directly to that court to decide all the issues in the case.

Appellant challenged state approval of the resident-hiring quota as ultra vires, and as unconstitutional under the Commerce Clause and the Privileges and Immunities Clause of Art. IV of the United States Constitution and under the Fourteenth Amendment’s Equal Protection Clause.5 The New Jersey court sustained the Treasurer’s action as consistent both with state law and the Federal Constitution. Citing Reeves, Inc. v. Stake, 447 U. S. 429 (1980), and Hughes v. Alexandria Scrap Corp., 426 U. S. 794 (1976), the court held that the resident quota was not subject to challenge under the Commerce Clause because the State was acting as a market participant rather than as a market regulator. 88 *213N. J., at 338-341, 443 A. 2d, at 158-160. The court also held that the quota did not violate the Privileges and Immunities Clause because it was not aimed primarily at out-of-state residents. “It almost certainly affects more New Jersey residents not living in Camden than it does out-of-state residents. Because the Camden ordinance does not affect The States^] . . . treatment of each other’s residents,’ ... it does not violate any privilege of state citizenship.” Id., at 341-342, 443 A. 2d, at 160. Finally, the New Jersey Supreme Court held that the 1-year residency requirement did not violate the right to travel protected by the Equal Protection Clause, concluding that only a rational basis is required to uphold a residency requirement for city employment. Id., at 342-343, 443 A. 2d, at 160-161.

Appellant then filed this appeal raising the same three constitutional challenges to the resident-hiring quota. We noted probable jurisdiction. 460 U. S. 1021 (1983). Since the Council filed its appeal, however, there have been two significant changes in the posture of the case. First, the Court decided White v. Massachusetts Council of Construction Employers, Inc., 460 U. S. 204 (1983), which held that an executive order of the Mayor of Boston, requiring that at least 50% of all jobs on construction projects funded in whole or in part by city funds be filled by bona fide city residents, was immune from scrutiny under the Commerce Clause because Boston was acting as a market participant rather than as a market regulator. In light of the decision in White, appellant has abandoned its Commerce Clause challenge to the Camden ordinance.

Second, in July 1983 Camden amended its affirmative-action plan. The 1-year residency requirement was deleted, thereby mooting appellant’s equal protection challenge based on that durational requirement. Now, a resident of the city of Camden is defined simply as “any person who resides in the City of Camden.” App. to Brief for Appellees Mayor and Council of the City of Camden A-5. Also, the scope of *214the ordinance was clarified.6 It now applies to any construction project “which is funded in whole or in part with City funds or funds which the City expends or administers in accordance with the terms of a grant.” Id., at A-4. Finally, the 40% resident-hiring requirement was changed from a strict “quota” to a “goal” with which developers and contractors must make “every good faith effort” to comply. Id., at A-13.

Because of these changes, the only question left for our consideration is whether the Camden ordinance, as now written, violates the Privileges and Immunities Clause.7 We first address the argument, accepted by the Supreme Court of New Jersey, that the Clause does not even apply to a municipal ordinance such as this. Two separate contentions are advanced in support of this position: first, that the Clause only applies to laws passed by a State and, second, that the Clause only applies to laws that discriminate on the basis of state citizenship.

The first argument can be quickly rejected. The fact that the ordinance in question is a municipal, rather than a state, law does not somehow place it outside the scope of the Privileges and Immunities Clause. First of all, one cannot easily distinguish municipal from state action in this case: the municipal ordinance would not have gone into effect without express approval by the State Treasurer. As the New Jersey Supreme Court noted in discussing the constitutionality of the minority hiring goals:

“By approving the Camden plan, the State Treasurer has established a minority hiring goal for the City of Camden that operates no differently than every other minority hiring goal established by the State Treas*215urer. . . . The Council’s constitutional challenge to the Camden minority hiring goal must therefore be interpreted as a challenge to the State Treasurer’s general power to issue affirmative action hiring goals.” 88 N. J., at 330, 443 A. 2d, at 154.

The constitutional challenge to the resident hiring preference, therefore, must also “be interpreted as a challenge to the State Treasurer’s general power” to adopt such a preference.8 The New Jersey court specifically found that the State Treasurer’s approval of the resident-hiring preference was “not ultra vires or an abuse of discretion.” Id., at 329, 443 A. 2d, at 154.

More fundamentally, a municipality is merely a political subdivision of the State from which its authority derives. Trenton v. New Jersey, 262 U. S. 182, 187 (1923). It is as true of the Privileges and Immunities Clause as of the Equal Protection Clause that what would be unconstitutional if done directly by the State can no more readily be accomplished by a city deriving its authority from the State. Memorial Hospital v. Maricopa County, 415 U. S. 250, 256 (1974); Avery v. Midland County, 390 U. S. 474, 480-481 (1968). Thus, even if the ordinance had been adopted solely by Camden, and not pursuant to a state program or with state approval, the hiring preference would still have to comport with the Privileges and Immunities Clause.

The second argument merits more consideration. The New Jersey Supreme Court concluded that the Privileges and Immunities Clause does not apply to an ordinance that discriminates solely on the basis of municipal residency. The Clause is phrased in terms of state citizenship and was designed “to place the citizens of each State upon the same footing with citizens of other States, so far as the advantages *216resulting from citizenship in those States are concerned.” Paul v. Virginia, 8 Wall. 168, 180 (1869). See also Hicklin v. Orbeck, 437 U. S. 518, 523-524 (1978); Ward v. Maryland, 12 Wall. 418, 430 (1871).

“The primary purpose of this clause, like the clauses between which it is located — those relating to full faith and credit and to interstate extradition of fugitives from justice — was to help fuse into one Nation a collection of independent, sovereign States. It was designed to insure to a citizen of State A who ventures into State B the same privileges which the citizens of State B enjoy. For protection of such equality the citizen of State A was not to be restricted to the uncertain remedies afforded by diplomatic processes and official retaliation.” Toomer v. Witsell, 334 U. S. 385, 395 (1948) (footnote omitted).

Municipal residency classifications, it is argued, simply do not give rise to the same concerns.

We cannot accept this argument. We have never read the Clause so literally as to apply it only to distinctions based on state citizenship. For example, in Mullaney v. Anderson, 342 U. S. 415, 419-420 (1952), the Court held that the Alaska Territory had no more freedom to discriminate against those not residing in the Territory than did any State to favor its own citizens. And despite some initial uncertainty, compare Travis v. Yale & Towne Mfg. Co., 252 U. S. 60, 78-79 (1920), and Blake v. McClung, 172 U. S. 239, 246-247 (1898), with Douglas v. New York, N. H. & H. R. Co., 279 U. S. 377, 386-387 (1929), and La Tourette v. McMaster, 248 U. S. 465, 469-470 (1919), it is now established that the terms “citizen” and “resident” are “essentially interchangeable,” Austin v. New Hampshire, 420 U. S. 656, 662, n. 8 (1975), for purposes of analysis of most cases under the Privileges and Immunities Clause. See Hicklin v. Orbeck, supra, at 524, n. 8; Toomer v. Witsell, supra, at 397. A person who is not residing in a given State is ipso facto not residing in a city within that *217State. Thus, whether the exercise of a privilege is conditioned on state residency or on municipal residency he will just as surely be excluded.

Given the Camden ordinance, an out-of-state citizen who ventures into New Jersey will not enjoy the same privileges as the New Jersey citizen residing in Camden. It is true that New Jersey citizens not residing in Camden will be affected by the ordinance as well as out-of-state citizens. And it is true that the disadvantaged New Jersey residents have no claim under the Privileges and Immunities Clause. Slaughter-House Cases, 16 Wall. 36, 77 (1873). But New Jersey residents at least have a chance to remedy at the polls any discrimination against them. Out-of-state citizens have no similar opportunity, Austin v. New Hampshire, supra, at 662, and they must not “be restricted to the uncertain remedies afforded by diplomatic processes and official retaliation.” Toomer v. Witsell, supra, at 395.9 We conclude that Cam*218den’s ordinance is not immune from constitutional review at the behest of out-of-state residents merely because some instate residents are similarly disadvantaged. Cf. Zobel v. Williams, 457 U. S. 55, 75 (1982) (O’Connor, J., concurring in judgment).

Application of the Privileges and Immunities Clause to a particular instance of discrimination against out-of-state residents entails a two-step inquiry. As an initial matter, the Court must decide whether the ordinance burdens one of those privileges and immunities protected by the Clause. Baldwin v. Montana Fish and Game Comm’n, 436 U. S. 371, 383 (1978). Not all forms of discrimination against citizens of other States are constitutionally suspect.

“Some distinctions between residents and nonresidents merely reflect the fact that this is a Nation composed of individual States, and are permitted; other distinctions are prohibited because they hinder the formation, the purpose, or the development of a single Union of those States. Only with respect to those ‘privileges’ and ‘immunities’ bearing upon the vitality of the Nation as a single entity must the State treat all citizens, resident and nonresident, equally.” Ibid.

As a threshold matter, then, we must determine whether an out-of-state resident’s interest in employment on public works contracts in another State is sufficiently “fundamental” to the promotion of interstate harmony so as to “fall within the purview of the Privileges and Immunities Clause.” Id., at 388. See also Canadian Northern R. Co. v. Eggen, *219252 U. S. 553, 560 (1920); Blake v. McClung, 172 U. S., at 248.

Certainly, the pursuit of a common calling is one of the most fundamental of those privileges protected by the Clause. Baldwin v. Montana Fish and Game Comm’n, supra, at 387. Many, if not most, of our cases expounding the Privileges and Immunities Clause have dealt with this basic and essential activity. See, e. g., Hicklin v. Orbeck, 437 U. S. 518 (1978); Austin v. New Hampshire, 420 U. S. 656 (1975); Mullaney v. Anderson, 342 U. S. 415 (1952); Toomer v. Witsell, 334 U. S. 385 (1948); Ward v. Maryland, 12 Wall. 418 (1871). Public employment, however, is qualitatively different from employment in the private sector; it is a subspecies of the broader opportunity to pursue a common calling. We have held that there is no fundamental right to government employment for purposes of the Equal Protection Clause. Massachusetts Bd. of Retirement v. Murgia, 427 U. S. 307, 313 (1976) (per curiam). Cf. McCarthy v. Philadelphia Civil Service Comm’n, 424 U. S. 645 (1976) (per curiam) (rejecting equal protection challenge to municipal residency requirement for municipal workers). And in White, 460 U. S., at 211, n. 7, we held that for purposes of the Commerce Clause everyone employed on a city public works project is, “in a substantial if informal sense, ‘working for the city.’”

It can certainly be argued that for purposes of the Privileges and Immunities Clause everyone affected by the Camden ordinance is also “working for the city” and, therefore, has no grounds for complaint when the city favors its own residents. But we decline to transfer mechanically into this context an analysis fashioned to fit the Commerce Clause. Our decision in White turned on a distinction between the city acting as a market participant and the city acting as a market regulator. The question whether employees of contractors and subcontractors on public works projects were or were not, in some sense, working for the city was crucial to that analysis. The question had to be answered in order to chart the boundaries of the distinction. But the distinction be*220tween market participant and market regulator relied upon in White to dispose of the Commerce Clause challenge is not dispositive in this context. The two Clauses have different aims and set different standards for state conduct.

The Commerce Clause acts as an implied restraint upon state regulatory powers. Such powers must give way before the superior authority of Congress to legislate on (or leave unregulated) matters involving interstate commerce. When the State acts solely as a market participant, no conflict between state regulation and federal regulatory authority can arise. White, supra, at 206-208; Reeves, Inc. v. Stake, 447 U. S., at 436-487; Hughes v. Alexandria Scrap Corp., 426 U. S., at 810. The Privileges and Immunities Clause, on the other hand, imposes a direct restraint on state action in the interests of interstate harmony. Hicklin v. Orbeck, supra, at 523-524; Ward v. Maryland, supra, at 430; Paul v. Virginia, 8 Wall., at 180. This concern with comity cuts across the market regulator-market participant distinction that is crucial under the Commerce Clause. It is discrimination against out-of-state residents on matters of fundamental concern which triggers the Clause, not regulation affecting interstate commerce. Thus, the fact that Camden is merely setting conditions on its expenditures for goods and services in the marketplace does not preclude the possibility that those conditions violate the Privileges and Immunities Clause.

In Hicklin v. Orbeck, supra, we struck down as a violation of the Privileges and Immunities Clause an “Alaska Hire” statute containing a resident-hiring preference for all employment related to the development of the State’s oil and gas resources.10 Alaska argued in that case that “because the oil and gas that are the subject of Alaska Hire are owned *221by the State, this ownership, of itself, is sufficient justification for the Act’s discrimination against nonresidents, and takes the Act totally without the scope of the Privileges and Immunities Clause.” Id., at 528 (footnote omitted). We concluded, however, that the State’s interest in controlling those things it claims to own is not absolute. “Rather than placing a statute completely beyond the Clause, a State’s ownership of the property with which the statute is concerned is a factor — although often the crucial factor — to be considered in evaluating whether the statute’s discrimination against noncitizens violates the Clause.” Id., at 529. See also Baldwin v. Montana Fish and Game Comm’n, 436 U. S., at 385. Much the same analysis, we think, is appropriate to a city’s efforts to bias private employment decisions in favor of its residents on construction projects funded with public moneys. The fact that Camden is expending its own funds or funds it administers in accordance with the terms of a grant is certainly a factor — perhaps the crucial factor — to be considered in evaluating whether the statute’s discrimination violates the Privileges and Immunities Clause. But it does not remove the Camden ordinance completely from the purview of the Clause.

In sum, Camden may, without fear of violating the Commerce Clause, pressure private employers engaged in public works projects funded in whole or in part by the city to hire city residents. But that same exercise of power to bias the employment decisions of private contractors and subcontractors against out-of-state residents may be called to account under the Privileges and Immunities Clause. A determination of whether a privilege is “fundamental” for purposes of that Clause does not depend on whether the employees of private contractors and subcontractors engaged in public works projects can or cannot be said to be “working for the city.” The opportunity to seek employment with such private employers is “sufficiently basic to the livelihood of the Nation,” Baldwin v. Montana Fish and Game Comm’n, supra, at 388, as to fall within the purview of the Privileges *222and Immunities Clause even though the contractors and subcontractors are themselves engaged in projects funded in whole or part by the city.

The conclusion that Camden’s ordinance discriminates against a protected privilege does not, of course, end the inquiry. We have stressed in prior cases that “[l]ike many other constitutional provisions, the privileges and immunities clause is not an absolute.” Toomer v. Witsell, 334 U. S., at 396. It does not preclude discrimination against citizens of other States where there is a “substantial reason” for the difference in treatment. “[T]he inquiry in each case must be concerned with whether such reasons do exist and whether the degree of discrimination bears a close relation to them.” Ibid. As part of any justification offered for the discriminatory law, nonresidents must somehow be shown to “constitute a peculiar source of the evil at which the statute is aimed.” Id., at 398.

The city of Camden contends that its ordinance is necessary to counteract grave economic and social ills. Spiralling unemployment, a sharp decline in population, and a dramatic reduction in the number of businesses located in the city have eroded property values and depleted the city’s tax base. The resident-hiring preference is designed, the city contends, to increase the number of employed persons living in Camden and to arrest the “middle-class flight” currently plaguing the city. The city also argues that all non-Camden residents employed on city public works projects, whether they reside in New Jersey or Pennsylvania, constitute a “source of the evil at which the statute is aimed.” That is, they “live off” Camden without “living in” Camden. Camden contends that the scope of the discrimination practiced in the ordinance, with its municipal residency requirement, is carefully tailored to alleviate this evil without unreasonably harming nonresidents, who still have access to 60% of the available positions.

Every inquiry under the Privileges and Immunities Clause “must... be conducted with due regard for the principle that *223the States should have considerable leeway in analyzing local evils and in prescribing appropriate cures.” Toomer v. Wit-sell, supra, at 396. This caution is particularly appropriate when a government body is merely setting conditions on the expenditure of funds it controls. See supra, at 221. The Alaska Hire statute at issue in Hicklin v. Orbeck, 437 U. S. 518 (1978), swept within its strictures not only contractors and subcontractors dealing directly with the State’s oil and gas; it also covered suppliers who provided goods and services to those contractors and subcontractors. We invalidated the Act as “an attempt to force virtually all businesses that benefit in some way from the economic ripple effect of Alaska’s decision to develop its oil and gas resources to bias their employment practices in favor of the State’s residents.” Id., at 531. No similar “ripple effect” appears to infect the Camden ordinance. It is limited in scope to employees working directly on city public works projects.

Nonetheless, we find it impossible to evaluate Camden’s justification on the record as it now stands. No trial has ever been held in the case. No findings of fact have been made. The Supreme Court of New Jersey certified the case for direct appeal after the brief administrative proceedings that led to approval of the ordinance by the State Treasurer. It would not be appropriate for this Court either to make factual determinations as an initial matter or to take judicial notice of Camden’s decay. We, therefore, deem it wise to remand the case to the New Jersey Supreme Court. That court may decide, consistent with state procedures, on the best method for making the necessary findings.

The judgment of the Supreme Court of New Jersey is reversed, and the case is remanded for proceedings not inconsistent with this opinion.

It is so ordered.

Justice Blackmun,

dissenting.

For over a century the underlying meaning of the Privileges and Immunities Clause of the Constitution’s Article *224IV1 has been regarded as settled: at least absent some substantial, noninvidious justification, a State may not discriminate between its own residents and residents of other States on the basis of state citizenship.2 See generally Hicklin v. Orbeck, 437 U. S. 518, 523-526 (1978); Toomer v. Witsell, 334 U. S. 385, 395 (1948); Hague v. CIO, 307 U. S. 496, 511 (1939) (opinion of Roberts, J.); Slaughter-House Cases, 16 Wall. 36, 77 (1873); Paul v. Virginia, 8 Wall. 168, 180 (1869).

Today, however, the Court casually extends the scope of the Clause by holding that it applies to laws that discriminate among state residents on the basis of municipal residence, simply because discrimination on the basis of municipal residence disadvantages citizens of other States “ipso facto.” Ante, at 216-217. This novel interpretation arrives accompanied by little practical justification and no historical or textual support whatsoever. Because I believe that the Privileges and Immunities Clause was not intended to apply to the kind of municipal discrimination presented by this case, I would affirm the judgment of the Supreme Court of New Jersey.3

HH

The historical underpinnings of the Privileges and Immunities Clause are not in serious dispute. The Clause was derived from the fourth Article of Confederation4 and was *225designed to carry forward that provision’s prescription of interstate comity. Austin v. New Hampshire, 420 U. S. 656, 660-661 (1975); United States v. Wheeler, 254 U. S. 281, 294 (1920); Slaughter-House Cases, 16 Wall., at 75. Both the text of the Clause and the historical record confirm that the Framers meant to foreclose any one State from denying citizens of other States the same “privileges and immunities” accorded its own citizens. See Austin v. New Hampshire, 420 U. S., at 660-661. James Madison complained during the Constitutional Convention of “Acts of Virga. & Maryland which give a preference to their own citizens in cases where the Citizens [of other States] are entitled to equality of privileges by the Articles of Confederation.”5 Alexander Hamilton, who deemed the Privileges and Immunities Clause “the basis of the Union,” The Federalist No. 80, p. 502 (B. Wright ed. 1961), expressly linked the Clause with the concern over state parochialism that gave rise to the federal courts’ diversity jurisdiction under Article III:

“[I]n order to [ensure] the inviolable maintenance of that equality of privileges and immunities to which the citizens of the Union will be entitled, the national judiciary ought to preside in all cases in which one State or its citizens are opposed to another State or its citizens. To secure the full effect of so fundamental a provision against all evasion and subterfuge, it is necessary that its construction should be committed to that tribunal which, having no local attachments, will be likely to be impartial *226between the different States and their citizens . . . Ibid.

While the Framers thus conceived of the Privileges and Immunities Clause as an instrument for frustrating discrimination based on state citizenship, there is no evidence of any sort that they were concerned by intrastate discrimination based on municipal residence. The most obvious reason for this is also the most simple one: by the time the Constitution was enacted, such discrimination was rarely practiced and even more rarely successful.6 Even had attempts to practice the kind of economic localism at issue here been more widespread, moreover, there is little reason to believe that the Framers would have devoted their limited institutional resources to bringing such conduct within the ambit of the Privileges and Immunities Clause. Whatever the weaknesses of the new state governments in suppressing sectional conflicts that gave rise to outright physical violence, like Shays’ Rebellion in 1786-1787, the States had more than adequate powers to prevent localities from disrupting the States’ internal economic affairs through discriminatory ordinances and regulations. By the time the Constitution was adopted, most state legislatures had assumed the power to grant and alter municipal charters and the power to legislate with respect to municipal affairs.7 Even before the Revolution, the colonial legislatures had shown themselves willing and able to exercise this authority to override local protectionist ordinances. In 1746, for example, the New York Assembly dismantled a cartel of New York City lawyers by requiring the city to open its Mayor’s Court to qualified lawyers from *227throughout the colony.8 As a result, the Framers had every reason to believe that intrastate discrimination based on municipal residence could and would be dealt with by the States themselves in those instances where it persisted.9

In light of the historical context in which the Privileges and Immunities Clause was adopted, it hardly is surprising that none of this Court’s intervening decisions has suggested that the Clause applies to discrimination on the basis of municipal residence. To the contrary, while the Court never has addressed the question directly,10 it repeatedly has proceeded on the assumption that the “Privileges and Immunities of Citizens” to which the Clause refers are entitlements held equally by all citizens of a State. Thus, in Paul v. Virginia, *228 supra, the Court stated that the Clause safeguards the enjoyment of “those privileges and immunities which are common to the citizens [in a State] under their constitution and laws by virtue of their being citizens.” 8 Wall., at 180. In Blake v. McClung, 172 U. S. 239 (1898), the Court condemned a Tennessee statute that granted a priority to resident creditors over nonresident creditors on the assumption that the State’s rules governing debtor-creditor relations “will be applied by its courts in all appropriate cases between citizens of that State, without making any distinction between them.” Id., at 254 (emphasis in original). In Travellers’ Insurance Co. v. Connecticut, 185 U. S. 364 (1902), the Court rejected a Privileges and Immunities Clause challenge to a Connecticut statute that taxed nonresident stockholders at a nominally higher rate than resident stockholders, on the ground that the direct differential was roughly offset by municipal taxes paid only by residents. The Court recognized that the burden borne by nonresidents might exceed that borne by residents in a particular year, but pointed out that “a like inequality will exist between residents of different localities in the State by reason of the different rates of taxation in those localities”; the disparate burden was permissible under these circumstances because “[y]ou cannot put one resident against one non-resident stockholder and by a comparison of their different burdens determine the validity of the legislation any more than you can place a stockholder resident in one municipality over against a stockholder resident in another municipality, and by comparison of their different burdens determine the validity of the tax law in respect to resident stockholders.” Id., at 369 (emphasis added). In each case, the underlying assumption has been that the constitutionality vel non of a particular statute under the Privileges and Immunities Clause turns on whether the statute deprives nonresidents of benefits enjoyed in common by state residents by virtue of their residence simpliciter.

Indeed, I had understood the Court to have reaffirmed this principle only two Terms ago in Zobel v. Williams, 457 U. S. *22955 (1982). In Zobel, the Court held that an Alaska statute which allocated state treasury refunds to state residents on the basis of the length of their residence violated the Equal Protection Clause. The Court declined, however, to hold that the statute violated the Privileges and Immunities Clause. It observed that the statute “does not simply make distinctions between native-born Alaskans and those who migrate to Alaska from other states;” instead, it “also discriminates among long-time residents and even native-born residents.” 457 U. S., at 59, n. 5. As a result:

“The statute does not involve the kind of discrimination which the Privileges and Immunities Clause of Art. IV was designed to prevent. That Clause ‘was designed to insure to a citizen of State A who ventures into State B the same privileges which the citizens of State B enjoy.’ Toomer v. Witsell, 334 U. S. 385, 395 (1948). The Clause is thus not applicable to this case.” Id., at 60, n. 5.

I am somewhat at a loss to understand how the Court’s decision today can be reconciled with its reasoning in Zobel. 11 The Alaska statute at issue in Zobel fell outside the scope of *230the Privileges and Immunities Clause for the elementary reason that it did not discriminate between state residents and nonresidents on the basis of state residence; rather, it discriminated among state residents in a way that disadvantaged nonresidents as well but did not thereby implicate the underlying concerns of the Privileges and Immunities Clause. The Camden ordinance presently before the Court occupies precisely the same position.

The Court’s decision clashes with other Privileges and Immunities Clause precedents as well. The Court recognizes, as it must, that the Privileges and Immunities Clause does not afford state residents any protection against their own State’s laws. See, e. g., Bradwell v. Illinois, 16 Wall. 130, 138 (1873); Slaughter-House Cases, 16 Wall., at 77. When this settled rule is combined with the Court’s newly fashioned rule concerning municipal discrimination, however, it has the perverse effect of vesting non-New Jersey residents with constitutional privileges that are not enjoyed by most New Jersey residents themselves. This result is directly contrary to the Court’s longstanding position that the Privileges and Immunities Clause does not give nonresidents “higher and greater privileges than are enjoyed by the citizens of the state itself.” Bank of Augusta v. Earle, 13 Pet. 519, 586 (1839); accord, Shaffer v. Carter, 252 U. S. 37, 53 (1920); Detroit v. Osborne, 135 U. S. 492, 498 (1890). When judicial alchemy transmutes gold into lead in this fashion, it is time for the Court to reexamine its reasoning.

Finally, the Court fails to attend to the functional considerations that underlie the Privileges and Immunities Clause. The Clause has been a necessary limitation on state autonomy not simply because of the self-interest of individual States, but because state parochialism is likely to go unchecked by state political processes when those who are disadvantaged are by definition disenfranchised as well. The Clause remedies this breakdown in the representative process by requiring state residents to bear the same burdens that they choose to place on nonresidents; “by constitution*231ally tying the fate of outsiders to the fate of those possessing political power, the framers insured that their interests would be well looked after.” J. Ely, Democracy and Distrust 83 (1980). As a practical matter, therefore, the scope of the Clause may be measured by asking whether failure to link the interests of those who are disadvantaged with the interests of those who are preferred will consign the former group to “the uncertain remedies afforded by diplomatic processes and official retaliation.” Toomer v. Witsell, 334 U. S., at 395; see Austin v. New Hampshire, 420 U. S., at 662.

Contrary to the Court’s tacit assumption, discrimination on the basis of municipal residence is substantially different in this regard from discrimination on the basis of state citizenship. The distinction is simple but fundamental: discrimination on the basis of municipal residence penalizes persons within the State’s political community as well as those without. The Court itself points out that while New Jersey citizens who reside outside Camden are not protected by the Privileges and Immunities Clause, they may resort to the State’s political processes to protect themselves. Ante, at 217. What the Court fails to appreciate is that this avenue of relief for New Jersey residents works to protect residents of other States as well; disadvantaged state residents who turn to the state legislature to displace ordinances like Camden’s further the interests of nonresidents as well as their own.12 *232Nor is this mechanism for relief merely a theoretical one; in the past decade several States, including California and Georgia, have repealed or forbidden protectionist ordinances like the one at issue here.13 In short, discrimination on the basis of municipal residence simply does not consign residents of other States, in the words of Toomer, supra, to “the uncertain remedies afforded by diplomatic processes and official retaliation.” The Court thus has applied the Privileges and Immunities Clause without regard for the political ills that it was designed to cure.14

*233It still might be possible to redeem the Court’s decision if it were compelled by the language of the Privileges and Immunities Clause. The Court itself, however, concedes that its interpretation of the Clause does not attach readily to a constitutional provision phrased solely in terms of state citizenship. Ante, at 216. The Court seeks to defend its excursion beyond the frontiers of the constitutional language on the ground that it never has read the Privileges and Immunities Clause literally to apply only to classifications based on state citizenship. Ibid. The examples it cites, however, are hardly compelling support. Mullaney v. Anderson, 342 U. S. 415 (1952), held not that the Privileges and Immunities Clause applies ex proprio vigore to discrimination by a territorial legislature based on territorial residence, but rather that Congress had made the Privileges and Immunities Clause applicable to the Territory of Alaska by statute. See 342 U. S., at 419-420.15 See also Haavik v. Alaska Packers *234 Assn., 263 U. S. 510, 515 (1924). Even if Mullaney v. Anderson set forth the proposition for which it is cited, moreover, the practical similarity between discrimination based on territorial residence and discrimination based on state residence has no parallel here. Similarly, while the Court unquestionably has come to treat the terms “citizen” and “resident” in this area as “essentially interchangeable,” Austin v. New Hampshire, 420 U. S., at 662, n. 8, it has done so not out of a general disregard for the Constitution’s language, but rather because the practical relationship between residence and citizenship is close enough that discrimination on the basis of the one criterion effectively amounts to discrimination based on the other. Cf. Travis v. Yale & Towne Mfg. Co., 252 U. S. 60, 79 (1920); Currie & Schreter, Unconstitutional Discrimination in the Conflict of Laws: Privileges and Immunities, 69 Yale L. J. 1323, 1344 (1960). These decisions are not, therefore, license for the Court to set aside the language of the Privileges and Immunities Clause as an inconvenient obstacle to a preferred result. Whenever this Court has departed from the literal language of the Clause in the past, it has remained faithful to the underlying purposes of the Clause. For the reasons already set forth, I believe that the Court’s decision today does not satisfy that requirement.

II

Needless to say, my view of the constitutional question in this case does not depend on my personal opinion about the desirability of the course on which Camden has embarked. I do not find “beggar thy neighbor” economic policies any more *235attractive when practiced by municipalities than when practiced by States or nations. The unedifying sight of localities fighting for parochial gain at one another’s expense gives new urgency to Benjamin Franklin’s reputed warning that “we must ... all hang together, or most assuredly we shall all hang separately.” R. Clark, Benjamin Franklin 286 (1983). At the risk of restating the obvious, however, the issue before us is not the desirability of the ordinance but its constitutionality — more particularly, its constitutionality under the Privileges and Immunities Clause.16 Because I believe that the Clause does not apply to discrimination based on municipal residence, I dissent.