Which military alliance was established to prevent Soviet aggression in Europe?

The North Atlantic Treaty Organization (NATO) was established in 1949 primarily to provide collective defense against potential aggression from the Soviet Union and its Eastern Bloc allies during the Cold War. By forming this military alliance, member countries agreed that an attack against one would be considered an attack against all, thus creating a united front aimed at deterring Soviet expansionism in Europe. The founding of NATO marked a significant commitment by Western nations to safeguard democracy and stability on the continent, aligning their military strategies and resources in the face of perceived threats from the Soviet-led alliance.

In contrast, the United Nations serves a broader purpose of promoting international cooperation and peace but does not function specifically as a military alliance. The Warsaw Pact was established later, in 1955, as a counterbalance to NATO among communist countries. SEATO (Southeast Asia Treaty Organization) was formed to prevent the spread of communism in Southeast Asia but is distinct from NATO's focus on Europe and was less effective in comparison. Thus, NATO's foundation was specifically geared towards addressing and containing Soviet aggression in Europe, making it the correct answer.