A major aspect of the Truman Doctrine, which was announced and implemented after the end of World War II, was the policy of containment. Truman believed that to keep a balance of power, maintain stability in international politics, foster democracy, and impede totalitarian states, it was imperative that communism be held back, or contained.
Truman announced his policy on March 12, 1947, in a speech to a joint session of Congress. The immediate impetus was to provide aid to the Greek government, which was fighting a civil war against the Greek Communist Party. However, Truman's containment policy also caused the United States to support the French administration in Indochina in light of Ho Chi Minh's declaration of independence of Vietnam from France. The US administration was concerned that a communist victory in Vietnam would cause communism to spread throughout Southeast Asia. This began the US involvement in the conflict in Vietnam, which would not end until the last evacuation of US personnel during the fall of Saigon on April 29, 1975.
Truman's policy of containment of the communist threat gave rise to a prevalent idea at the time known as the Domino Theory, which postulated that when one country fell under communist control, the other countries around it would follow. Belief in the Domino Theory caused a drastic escalation of the Vietnam War during the presidencies of John F. Kennedy and Lyndon B. Johnson. The policy of containment exemplified by the Truman Doctrine and the Domino Theory was also the motivation for US involvement in the Korean War. The Truman administration was concerned that if the North Korean communists overran South Korea, the security of Japan would be imperiled.