The so‐called Truman Doctrine was enunciated by President Harry S Truman in a speech to a joint session of the US Congress on 12 March 1947. In it he denounced the oppressive nature of the communist system of government and warned against the possibility that campaigns of subversion might bring even more countries under that system. He sought, and was given, Congressional authority to provide assistance to threatened regimes—initially those in Greece and Turkey. The ‘Doctrine’ was thus the starting point for the strategy of containment of communism developed by successive US Presidents during the Cold War.
http://www.yale.edu/lawweb/avalon/trudoc.htm Truman's speech of 12 March 1947.