What organization was created to prevent Soviet expansion in Europe?

The correct response highlights the formation of the North Atlantic Treaty Organization (NATO) as a key initiative established in response to concerns about Soviet expansion in Europe. Formed in 1949, NATO was a military alliance consisting of North American and European countries that aimed to provide collective defense against potential aggression, particularly from the Soviet Union during the Cold War. The principle of collective defense is central to NATO's framework, as outlined in Article 5 of the North Atlantic Treaty, where an attack against one member is considered an attack against all members. This alliance was crucial for deterring Soviet influence and military actions in Europe, thereby maintaining the balance of power in the region during a time of heightened tensions between the Western Allies and the Soviet Bloc.

Other organizations mentioned, such as the League of Nations, were focused on promoting peace and cooperation but did not specifically address the threat of Soviet expansion. The European Union, established later, primarily aims at economic and political integration among its member states rather than military alliances. The United Nations serves a different purpose of fostering international cooperation and peacekeeping but was not created specifically as a response to the threat of Soviet military expansion in Europe.