A treaty is null and void if it is in violation of a peremptory norm. These norms, unlike other principles of customary law, are recognized as permitting no violations and so cannot be altered through treaty obligations. These are limited to such universally accepted prohibitions as those against the aggressive use of force, genocide and other crimes against humanity, piracy, hostilities directed at civilian population, racial discrimination and apartheid, slavery and torture, meaning that no state can legally assume an obligation to commit or permit such acts.

What will a treaty be if it is in violation of a peremptory norm?