Russia’s war of aggression against Ukraine violates several ius cogens norms. This article explores what role the legal consequences of ius cogens play in this case, and, conversely, what implications international practice in response to the war has for ius cogens. 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 other concerning state responsibility. The first set should retain a limited role, so as not to invalidate a potential peace settlement (II.). Rather, the second set is crucial for protecting ius cogens norms against violations. It includes the obligations of non-recognition and non-assistance in situations created by serious ius cogens breaches, and the crystallising obligation to cooperate to end such breaches. This article analyses these obligations (III.), and concludes that, despite Russia’s ongoing aggression, international practice responding to the war reinforces ius cogens (IV.).
