Peremptory Norms of General International Law (Ius Cogens) and Russia’s Aggression Against Ukraine

Russia’s war against Ukraine violates several ius cogens norms, especially the prohibition of aggression (I.). Though ius cogens is said to protect fundamental values of the international community, it is not immediately apparent how it provides protection against an aggressor state. The article therefore analyses the implications of ius cogens for Ukraine, Russia, and the international community. The legal consequences of ius cogens can be divided in two sets, one concerning the invalidating effect of ius cogens on conflicting legal acts, the second concerning state responsibility. The first set raises the pivotal question under what circumstances a potential Russo-Ukrainian peace settlement would be invalid (II.). The second set of legal consequences engages the international community by conferring obligations on all states in relation to the war (III.). This includes the obligations of non-recognition and nonassistance in situations created by serious ius cogens breaches, and the obligation to cooperate to end such breaches. While the customary status and content of these obligations is not fully settled, state practice in response to the war contributes to crystallizing and clarifying these obligations to some extent. Therefore, despite Russia’s ongoing aggression, international practice responding to the war reinforces ius cogens (IV.).