The end of World War II in 1945 brought the rise of two great superpowers, the United States and the Soviet Union. The opposing ideologies of these two nations set the stage for the conflict known as the Cold War. For the next forty-five years, democratic powers led by the United States attempted to contain the worldwide spread of communism and the influence of the Soviets. American leaders met with varying degrees of success as they employed a range of methods to limit communist control, including the formation of alliances, the threat of nuclear weapons or other military force, covert military opposition, efforts at diplomacy and cooperation, and confrontation with the Soviets.
The U.S. had traditionally been reluctant to involve itself in world affairs; this isolationist policy began to disappear after the war, when it became evident that America, as the strongest democratic nation, needed to play an active role in global affairs in order to protect its own interests as well as those of its allies. President Truman took the first steps in this direction in 1947 with the Truman Doctrine, which declared that "totalitarian regimes imposed on free peoples, by direct or indirect aggression, undermine the foundations of international peace and hence the security of the United States."" This initiated the American policy of containment, which was reinforced by the formation of the North Atlantic Treaty Organization in 1949; NATO provided for the collective security of the United States and Western Europe against any attack. American resolve to uphold this peacetime alliance faced a major test during the Truman administration, with the Soviet blockade of West Berlin. An attempt to establish Soviet control in Germany and to weaken U.S. influence in Western Europe, the crisis turned out to be an American victory in which Truman clearly demonstrated that the United States had a permanent interest in containing the spread of communism.