Article 11(2) of the Universal Declaration of Human Rights states that
"No one shall be held guilty of any penal offence on account of any act or omission which did not constitute a penal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the penal offence was committed." This is referred to as the principle of legality.
This passage from Beth van Schaack ()who has frequently taught international human rights-related courses here at Stanford) elaborates on the idea:
One of the most fundamental defenses to a criminal prosecution is that of nullum crimen sine lege, nulla poena sine lege (“no crime without law, no punishment without law”). In its simplest translation, this Latin maxim asserts the ex post facto prohibition: that conduct must be criminalized and penalties fixed in advance of any criminal prosecution. More broadly, the maxim is also invoked in connection with corollary legislative and interpretive principles compelling criminal statutes to be drafted with precision (the principle of specificity), to be strictly construed without extension by analogy, and to have ambiguities resolved in favor of the accused (the principle of lenity or in dubio pro reo). Together, these precepts undergird the principle of legality and serve several purposes: ensuring that individuals are capable of obtaining notice of prescribed conduct so they can rationally adjust their behavior to avoid sanction; protecting the citizenry from arbitrary or oppressive state action in the face of ambiguities or gaps in the law; and effectuating the expressive purposes of the law by clearly articulating conduct that is collectively condemned. The principle of nullum crimen sine lege (NCSL) writ large thus embodies “an essential element of the rule of law” by speaking to the very legitimacy of a legal rule, providing a check on the power of all branches of government over individuals, and policing the separation of powers by ensuring legislative primacy in substantive rulemaking. Indeed, Alexander Hamilton recognized violations of the principle as “the favorite and most formidable instruments of tyranny.”
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