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The Truman Doctrine was a proclamation by U.S. president Harry S. Truman on March 12, 1947. It stated that the U.S. would support Greece and Turkey with economic and military aid to prevent their falling into the Soviet sphere. The Doctrine shifted American foreign policy as regards the Soviet Union from Détente to, as George F. Kennan phrased it, a policy of containment of Soviet expansion. Historians often use it to mark the starting date of the Cold War.