After the end of World War II and the end of the West-East bloc's cooperation in defeating the Allies, the rivalry between the two poles took a new form. Destroyed Europe came under American protection, and the Communist Soviet Union consolidated its influence to the gates of Berlin. The North Atlantic Treaty Organization was formed to protect the Western alliance against the advance of communism, although it played a fundamentally deterrent role and did not actually enter military operation until the end of the Cold War. But with the collapse of the Soviet Union, NATO faced serious substantive questions. Does its existence have anything to do with the revival of European allies after World War II, as well as the elimination of its communist rival? However, the founders stressed the need to maintain this security and defense institution, and since then, ie for three decades, NATO's geographical and thematic scope has increased and, of course, it has actually entered into several operations. But what is clear is that NATO's transition period is still going on, and there are debates and analyzes among politicians and strategists. Meanwhile, the formation and development of the European Union and the establishment of its institutions have made the issue of European defense and security a central issue for the pioneers of the idea of a united Europe, and this perspective is vital for a regional power like Iran. In these circumstances, the recent developments (Brexit, election of Donald Trump, terrorism and Middle East immigrants, cyber security, Russian movements and the emergence of extremism) necessitate effective action to shape the European Defense Organization and determine the scope of NATO's mission and performance. More than ever. In this study, using library and documentary sources and explanatory-analytical methods to study the existing trends and opinions, the perspective of this relationship is defined in the framework of institutional security cooperation approach and relevant scenarios and related challenges are presented. Since dependence on the United States is defined in the form of NATO, the dominant European view in this institution is essentially to maintain the scope of action within the borders of the North Atlantic Ocean, while the US approach is to expand the geographical area and subject matter of NATO missions.