The Truman Doctrine was an American foreign policy to stop Soviet imperialism during the Cold War. It was announced to Congress by President Harry S. Truman on March 12, 1947 when he pledged to contain Soviet threats to Greece and Turkey. No American military force was involved; instead Congress appropriated a free gift of financial aid to support the economies and the militaries of Greece and Turkey. More generally, the Truman doctrine implied American support for other nations threatened by Soviet communism. The Truman Doctrine became the foundation of American foreign policy, and led in 1949 to the formation of NATO, a full-fledged military alliance that is in effect to this day. 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 reasoned, because these "totalitarian regimes" coerced "free peoples", they represented a threat to international peace and the national security of the United States. Truman made the plea amid the crisis of the Greek Civil War (1946–49). He argued that if Greece and Turkey did not
receive the aid that they urgently needed, they would inevitably fall to communism with grave consequences throughout the region. Because Turkey and Greece were historic rivals, it was necessary to help both equally, even though the threat to Greece was more immediate. Eric Foner argues, the Truman 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 Britain had supported Greece, but was now near bankruptcy and was forced to radically reduce its involvement. In February 1947, Britain formally requested the United States take over its role in supporting the 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 Communist threat, and in 1952 both Greece and Turkey joined NATO, a military alliance that guaranteed their protection.
The 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 détente (a relaxation of tension) to a policy of containment of Soviet expansion as advocated by diplomat George Kennan. It avoided the policy of rollback because it implicitly tolerated the previous Soviet takeovers in Eastern Europe.