What was the purpose of the Truman Doctrine?

With the spread communism and the threat posed by the Soviet Union's increasing sphere of influence, President Truman established the Truman Doctrine in March 1947, by shifting U.S. foreign policy away from isolationism toward containment, specifically ceasing the spread of communism. 

The U.S. first intervened in Greece stopping a communist uprising. Other interventions included the Korean War and the Vietnam War.