The Truman Doctrine reoriented U.S. foreign policy, from its usual stance of withdrawal from conflicts not directly involving the United States, to one of intervention in far away conflicts.
The Truman Doctrine arose from a speech delivered by President Truman at Congress on March 12, 1947. The cause for the speech was a recent announcement by the British Government that, it would no longer provide military and economic assistance to the Greek Government in its civil war against the Greek Communist Party. Truman asked Congress to support the Greek Government against the Communists.
At the time, the U.S. Government believed that the Soviet Union supported the Greek Communist war effort and worried that if the Communists won the Greek civil war, the Soviets would ultimately influence Greek policy.