The U.S. joined NATO (North Atlantic Treaty Organization) in 1949. Established on April 4 of that year, NATO was created as a collective defense alliance among Western nations in response to the growing threat of Soviet expansion and communist influence during the early Cold War. The principle of collective defense under Article 5 of the NATO treaty means that an attack against one member is considered an attack against all. The U.S. played a key role in the formation and leadership of NATO, solidifying military alliances among Western nations as a deterrent against potential aggression from the Soviet Union.