The Beginning of the Cold War
The Cold War is not a "war" in the traditional sense but rather a competition between the United States, as the representative of Democracy and Capitalism, and the Soviet Union which represented the ideals of Communism. The competition centered on which ideology was better and more effective. At the end of World War II, these two countries were the sole superpowers of the world and each wanted to be the only superpower. This competition lasted for about 45 years between 1945 and 1991.
The Marshall Plan and the Truman Doctrine
Between 1823 and 1941, American foreign policy was defined by the Monroe Doctrine which established the policy of isolationism. After World War II, American foreign policy is redefined by the Truman Doctrine. President Truman, in his speech to Congress, stated that Communism must be contained and thus, the policy of "containment" was established.
President Truman's Secretary of State developed a plan to rebuild Europe after the War. He offered financial assistance to countries in Europe but that financial assistance would only be provided for Democratic nations. Any communist nation would not receive any assistance from the United States. In 1946, Winston Churchill stated that an "Iron Curtain" had descended upon Europe with the center of that division in Germany. Germany was split between Democratic West and Communist East.