Alien Enemies Act

On the history of the Act, from Justice Kagan's dissent in Trump v. JGG:

This case arises out of the President's unprecedented peacetime invocation of a wartime law known as the Alien Enemies Act. See Act of July 6, 1798, ch. 66, 1 Stat. 577. Enacted in 1798 by a Congress consumed with fear of war with France, the Alien Enemies Act provided a wartime counterpart to the widely denounced Alien Friends Act, which granted the President sweeping power to detain and expel any noncitizen he deemed “dangerous to the peace and safety of the United States.” Act of June 25, 1798, 1 Stat. 571. Unlike the Alien Friends Act, which lapsed in disrepute as James Madison deemed it “a monster that must for ever disgrace its parents,” the Founders saw the Alien Enemies Act as a constitutional exercise of Congress's powers to “declare War,” to “raise and support Armies,” and to “provide for calling forth the Militia to ... suppress Insurrections and repel Invasions.” U. S. Const., Art. I, § 8, cls. 11–15.¹

To that end, the Act grants the President power to detain and remove foreign citizens of a “hostile nation or government” when “there is a declared war” with such nation or when a “foreign nation” threatens “invasion or predatory incursion” against the territory of the United States. Rev. Stat. § 4067, 50 U.S.C. § 21. Before today, U. S. Presidents have invoked the Alien Enemies Act only three times, each in the context of an ongoing war: the War of 1812, World War I, and World War II.