The United States and 11 other nations establish the North Atlantic Treaty Organization (NATO), a mutual defense pact aimed at containing possible Soviet aggression against Western Europe. NATO stood as the main U.S.-led military alliance against the Soviet Union throughout the duration of the Cold War.

Relations between the United States and the Soviet Union began to deteriorate rapidly in 1948. There were heated disagreements over the postwar status of Germany, with the Americans insisting on German recovery and eventual rearmament and the Soviets steadfastly opposing such actions. In June 1948, the Soviets blocked all ground travel to the American occupation zone in West Berlin, and only a massive U.S. airlift of food and other necessities sustained the population of the zone until the Soviets relented and lifted the blockade in May 1949. In January 1949, President Harry S. Truman warned in his State of the Union Address that the forces of democracy and communism were locked in a dangerous struggle, and he called for a defensive alliance of nations in the North Atlantic—U.S military in Korea.NATO was the result. In April 1949, representatives from Belgium, Canada, Denmark, France, Great Britain, Iceland, Italy, Luxembourg, the Netherlands, Norway, and Portugal joined the United States in signing the NATO agreement. The signatories agreed, “An armed attack against one or more of them… shall be considered an attack against them all.” President Truman welcomed the organization as “a shield against aggression.”

Not all Americans embraced NATO. Isolationists such as Senator Robert A. Taft declared that NATO was “not a peace program; it is a war program.” Most, however, saw the organization as a necessary response to the communist threat. The U. S. Senate ratified the treaty by a wide margin in June 1949. During the next few years, Greece, Turkey, and West Germany also joined. The Soviet Union condemned NATO as a warmongering alliance and responded by setting up the Warsaw Pact (a military alliance between the Soviet Union and its Eastern Europe satellites) in 1955.

NATO lasted throughout the course of the Cold War, and continues to play an important role in post-Cold War Europe. In recent years, for example, NATO forces were active in trying to bring an end to the civil war in Bosnia.