NATO has gained increasing importance, particularly in light of its expansion into Eastern Europe, which prompted a Russian reaction in the form of the annexation of Crimea in 2014 and subsequent military action in the Russian Ukrainian war. Despite Ukraine’s official request to join, NATO has not yet made a decision on membership, and Ukraine remains only a partner nation. The ongoing war has sparked substantial discussions about NATO’s role in the conflict, including the position of Eastern European countries against Russian drone flyovers. While some members deem their engagement against Russia too risky, others advocate more direct measures, such as increasing air policing missions and the number of forward defence battalions. The following paper discusses the multi-faceted process of NATO enlargement in the context of Russian Ukrainian war which changed parameters of the security situation in Europe. The hypothesis of this study is that “What is the role of NATO and its expansion policies in Russia’s full-scale invasion of Ukraine?” The main theme of this study is to highlight why Russia perceived NATO’s expansion as a threat, and then why it invaded a part of Ukraine territory based on this threat assessment, and why some European countries subsequently felt compelled to act collectively under the NATO’s umbrella against Russia. In this study, qualitative research method, particularly text/document analysis method is used by conducting scientific research on reports released after the latest NATO summits, war reports related to the topic, articles on Russian Ukrainian war and NATO enlargement process.