Containment of the Soviet Union became American policy in the postwar years. George Kennan, a top official at the U.S. embassy in Moscow, defined the new approach in a long telegram he sent to the State Department in 1946. He extended his analysis after he returned home in an article published under the signature "X" in the prestigious journal Foreign Affairs. Pointing to Russia's traditional sense of insecurity, Kennan argued that the Soviet Union would not soften its stance under any circumstances. Moscow, he wrote, was "committed fanatically to the belief that with the U.S. there can be no permanent modus vivendi, that it is desirable and necessary that the internal harmony of our society be disrupted." Moscow's pressure to expand its power had to be stopped through "firm and vigilant containment of Russian expansive tendencies...."

The first significant application of the containment doctrine came in the eastern Mediterranean. Great Britain had been supporting Greece, where communist forces threatened the ruling monarchy in a civil war, and Turkey, where the Soviet Union pressed for territorial concessions and the right to build naval bases on the Bosporus. In 1947 Britain told the United States that it could no longer afford such aid. Quickly, the U.S. State Department devised a plan for U.S. assistance. But support for a new interventionist policy, Senate leaders such as Arthur Vandenberg told Truman, was only possible if he was willing to start "scaring the hell out of the country."

Truman was prepared to do so. In a statement that came to be known as the Truman Doctrine, he declared, "I believe that it must be the policy of the United States to support free peoples who are resisting subjugation by armed minorities or by outside pressures." To that end he asked Congress to provide $400 million for economic and military aid to Greece and Turkey, and the money was appropriated.

However, there was a price Truman himself and American society paid for his victory. To whip up American support for the policy of containment, Truman overstated the Soviet threat to the United States. In turn, his statements inspired a wave of hysterical anti-communism throughout the country and set the stage for the emergence of McCarthyism.

Containment also called for extensive economic aid to assist the recovery of war-torn Western Europe. With many of the region's nations economically and politically unstable, the United States feared that local communist parties, directed by Moscow, would capitalize on their wartime record of resistance to the Nazis and come to power. Something needed to be done, Secretary of State George Marshall noted, for "the patient is sinking while the doctors deliberate." Marshall was formerly the highest ranking officer in the U.S. armed forces and credited as the chief organizer of the American military victory in World War II. In mid-1947 Marshall asked troubled European nations to draw up a program "directed not against any country or doctrine but against hunger, poverty, desperation and chaos." The Soviets participated in the first planning meeting, then departed rather than share economic data on their resources and problems, and submit to Western controls on the expenditure of the aid. The remaining 16 nations hammered out a request that finally came to $17 thousand million for a four-year period. In early 1948 Congress voted to assist European economic recovery, dubbed the "Marshall Plan," and generally regarded as one of the most successful U.S. foreign policy initiatives in history.

Postwar Germany was divided into U.S., Soviet, British and French zones of occupation, with the former German capital of Berlin (itself divided into four zones), near the center of the Soviet zone. The United States, Britain and France had discussed converting their zones into a single, self-governing republic. But the Soviet Union opposed plans to unite Germany and ministerial-level four-power discussions on Germany broke down. When the Western powers announced their intention to create a consolidated federal state from their zones, Stalin responded. On June 23, 1948, Soviet forces blockaded Berlin, cutting off all road and rail access from the West.

American leaders feared that losing Berlin was but a prelude to losing Germany and subsequently all of Europe. Therefore, in a successful demonstration of Western resolve known as the Berlin Airlift, Allied air forces took to the sky, flying supplies into Berlin. U.S., French and British planes delivered nearly 2,250,000 tons of goods, including food and coal. Stalin lifted the blockade after 231 days and 277,264 flights.

Soviet domination of Eastern Europe alarmed the West. The United States led the effort to create a military alliance to complement economic efforts at containment. In 1949 the United States and 11 other countries established the North Atlantic Treaty Organization (NATO), an alliance based on the principle of collective security. An attack against one was to be considered an attack against all, to be met by appropriate force.

The next year, the United States defined its defense aims clearly. The National Security Council (NSC) undertook a full-fledged review of American foreign and defense policy. The resulting document, known as NSC-68, signaled a new direction in American security policy. Based on the assumption that "the Soviet Union was engaged in a fanatical effort to seize control of all governments wherever possible," the document committed America to assist allied nations anywhere in the world which seemed threatened by Soviet aggression. The United States proceeded to increase defense spending dramatically in response to Soviet threats against Europe and the American, British and French presence in West Berlin.