The Russian Federation’s unprovoked and unjustified war against Ukraine has changed the entire security environment, which will have long-term threatening consequences for the entire collective security system. The issue of forming an effective system of collective security is relevant not only for the international community, which is explained by its absolute importance in maintaining international law and order, but also for Ukraine in order to restore its state sovereignty and territorial integrity in countering Russian armed aggression. The purpose of the article is to clarify the essence of the collective security system, to analyze the activities of the most influential international security organizations (UN, NATO and OSCE) since the outbreak of the Russian-Ukrainian war and to assess their ability to end this war, to formulate problematic issues regarding the functioning of the existing collective security system, and to argue for the need to build a new collective security system with a definition of Ukraine's role and place in it. The conclusion of the study is that the modern system of collective security should be understood as a state of international relations that excludes violation of the general peace, threats to the security of states and peoples, and counteraction to acts of aggression, implemented by joint efforts of participating states at the global or regional levels. The collective security system is usually implemented through the activities of the UN, NATO and the OSCE in maintaining international law and order. However, in the context of numerous conflicts, including the Russian-Ukrainian war, there are more and more complaints about the activities of the UN, in particular the UN Security Council as the main body responsible for maintaining international peace and security. At the same time, the role of the UN should not be minimized, as the documents adopted by it are important for improving the collective security system and the international mechanism for bringing the aggressor state to justice. Despite the significant resistance of the Russian Federation, the UN, NATO and the OSCE strongly condemn Russia’s aggressive war against Ukraine and continue to provide our country with comprehensive support.