The article aims to address the following question: in the case of a war in Ukraine, is public international law an obstacle to the application of combined international enforcement action within the framework of the collective security system under the auspices of the United Nations, or whether such impediments lie elsewhere? Russia’s presence in the Security Council as a permanent member, and thus endowed with the privilege of vetoing resolutions, paralyses this body. Therefore, the subject of the analysis is what other actions of the United Nations are permitted by law. It is important for assessing the status of the UN as a collective actor in international relations, the main objective of which, under Art. 1 of the UN Charter, is “to maintain international peace and security, and, to that end: to take effective collective measures for (…) the suppression of acts of aggression”. Bearing in mind the fact that international law is a consensual legal order, the article assesses its available compulsory mechanisms and instruments. The supplementary objective is to determine whether it is permissible to use the term “war” with regard to the armed conflict between Russia and Ukraine.