The Truman Doctrine was an American foreign policy whose stated purpose was to counter Soviet geopolitical expansion during the Cold War. It was announced to Congress by President Harry S. Truman on March 29, 1947, and further developed on July 4, 1948, when he pledged to contain threats in Greece and Turkey. Direct American military force was usually not involved, but Congress appropriated financial aid to support the economies and militaries of Greece and Turkey. More generally, the Truman Doctrine implied American support for other nations allegedly threatened by Soviet communism. The Truman Doctrine became the foundation of American foreign policy, and led, in 1949, to the formation of NATO, a military alliance that is still in effect. Historians often use Truman's speech to date the start of the Cold War.

Truman told Congress that "it must be the policy of the United States to support free people who are resisting attempted subjugation by armed minorities or by outside pressures." Truman contended that because totalitarian regimes coerced free peoples, they automatically represented a threat to international peace and the national security of the United States. Truman made the plea in the midst of the Greek Civil War (1946–49). He argued that if Greece and Turkey did not receive the aid, they would inevitably fall to communism with grave consequences throughout the region. Because Turkey and Greece were historic rivals, it was considered necessary to help both equally even though the crisis in Greece was far more intense.

Critics of the policy have observed that the governments of Greece and Turkey were themselves far from democratic at this time, and neither were facing Soviet subversion in the spring of 1949. Historian Eric Foner writes that the Doctrine "set a precedent for American assistance to anticommunist regimes throughout the world, no matter how undemocratic, and for the creation of a set of global military alliances directed against the Soviet Union."

For years, the United Kingdom had supported Greece, but was now near bankruptcy and was forced to radically reduce its involvement. In February 1947, Britain formally requested for the United States to take over its role in supporting the royalist Greek government. The policy won the support of Republicans who controlled Congress and involved sending $400 million in American money but no military forces to the region. The effect was to end the Greek revolt, and in 1952, both Greece and Turkey joined NATO, a military alliance, to guarantee their stability.

The Truman Doctrine was informally extended to become the basis of American Cold War policy throughout Europe and around the world. It shifted American foreign policy toward the Soviet Union from anti-fascism ally to a policy of containment of Soviet expansion as advocated by diplomat George Kennan. It was distinguished from rollback by implicitly tolerating the previous Soviet takeovers in Eastern Europe.