The publication examines the legal nature of wars, looks into the law of war genesis, reviews its conventions, as well as identifies the aim, objectives, causes together with the consequences of Russia’s military aggression on February 24, 2022; it provides a testimony on the war of aggression against Ukraine and identifies its threats to post-war international law and order. The aim of the article is to determine the background, nature, fundamentals and nature of the war the Russian Federation launched on February 24, 2022 against Ukraine within the context of confrontation with the values of Western democracy. Historical-legal dialectical, comparative-legal and system-structural methods, as well as methods of formal logic (induction and deduction, analysis and synthesis, abstraction and concretization, etc.) were used to study the issues. The application of these research methods entails a cross-sectoral scientific nature. Wars have accompanied the entire history of mankind. Since the 17th century the theoretical foundations of the law of war have been laid, which enshrined in international laws and customs of warfare (Geneva Conventions, Hague Convention, etc.) since the 19th century, thus forming the international humanitarian law. After World War II, violations of the latter by the belligerent party are viewed by the world community no less harshly than the mere war. The paper questions the purpose, reasons, conditions and intermediate consequences of the war of the Russian Federation against Ukraine. Obviously, the military aggression and the Russian invasion on February 24, 2022 marked the beginning of the de facto war, under a specific formal definition by a Russian official as a “special military operation” instead of naming it an attempt to avoid legal responsibility for war crimes committed in Ukraine. The article analyses the aggressive nature of this war on the part of the Russian Federation, demonstrates numerous violations of international law conventions by the Russian armed forces during the military invasion and temporary occupied territories in Ukraine. It emphasizes that the law of war has always been replaced by signing a peace treaty and relevant international agreements, which entails the obligation to severe the political, economic, and legal responsibility of the state violating international humanitarian law. The paper reviews the key institutions and legal tools for bringing the Russian Federation authorities to justice for the war crimes committed in Ukraine. So far, the President of Ukraine, the Supreme Council of Ukraine, and the Cabinet of Ministers of Ukraine have managed to apply legitimate international legal mechanisms, primarily the International Criminal Court and the UN Security Council, to prosecute Russia for violating the international humanitarian law and committing war crimes in Ukraine. At the same time, on the Ukrainian territories currently liberated from the Russian army’s occupation (Bucha, Gostomel, Irpin, etc.), it is already possible to implement a special mechanism of justice, which consists in the collaboration between national and international experts: specialists, investigators, prosecutors, and judges to collect objective and impartial evidence of gross violations of international law and war conventions in Ukraine. It is obvious that for Ukraine today both the victory and just punishment for the committed and continuing committed war crimes on its territory are important. The aftermath of World War II’s “Never Again” now requires, as never before, its effective guarantee and protection