North Atlantic Treaty Organization was founded with the purpose of containing and deterring the Soviet Union. The enormous threat it posed has gone away. Russia does not pose a similar threat. Thus, the following problem has arisen: is NATO still indispensable and if so, what kind of NATO is needed in the 21st century? In other words: what is NATO’s raison d’être today? Analysis of various views and opinions raised on these issues is the main purpose of this article. It gives the most attention to different standpoints of Americans and their allies in Europe and their conflicting relations, which pose a danger to the unity and future of the Alliance. Another focus is the relation between NATO enlargement and its changing nature. The Alliance’s ability to respond to new challenges to international security, whose symbol became September II, and its role in the new global security system are explored as well.