The Truman Doctrine was the American policy in 1947 of providing economic and military aid to Greece and Turkey because they were threatened by communism. It was the start of the containment policy to stop Soviet expansion; it was a major step in beginning the Cold War.

President Harry S. Truman told Congress the Doctrine was "the policy of the United States to support free peoples 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 (1946–1949). 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.