By now, the principle of legality has come to play a central role in international practice of criminal justice that it did not play before World War II. Even in international criminal law, the rule nullum crimen, nulla poena sine lege has obtained customary international law status. Whether or not the entire rule of nullum crimen, nulla poena sine lege has become jus cogens, the principle of legality is a principle of justice whose enforcement is vital to the rule of law. No national, international, or comparative criminal jurisprudence can ignore it.

The exercise of criminal jurisdiction in violation of the rule of non-retroactivity of crimes and punishments is a violation of an international human right of the accused. The principle of legality implicates jurisdiction, as it concerns the unfairness of both retroactive definition of crimes and penalties (jurisdiction to prescribe) and retroactive application of these rules (jurisdiction to adjudicate and enforce). This is the real answer to those who argue (with some force) that issues of retroactivity have traditionally been considered separately from issues labeled “jurisdictional.” The principle also implicates the more traditionally jurisdictional issue of whether a court has been created by law.

There are those who argue that the Nazi mass murders, of the Roma as well as the Jews, made retroactive crime creation at Nuremberg acceptable. This is the force of the statement that it would have been “unjust” to let the Nazi leaders escape.