Why did the U.S. get involved in Korea?

The United States got involved in the Korean War to stop the communist invasion of South Korea by North Korea on June 25, 1950, and to demonstrate the credibility of the new American policy of containment. American leaders believed that if the United States allowed North Korea to absorb South Korea by force, the Soviet Union and its allies would conclude that they could expand communism with little resistance, leading to further aggression in Europe and Asia.

The decision had several specific drivers. First, the United States had occupied southern Korea after Japan's surrender in 1945 and helped establish the Republic of Korea in 1948 under Syngman Rhee. Although American combat troops had withdrawn in 1949, the United States retained moral and political responsibility for the new government.

Second, in February 1950 the Soviet Union and the new People's Republic of China signed a Treaty of Friendship, Alliance, and Mutual Assistance, deepening fears of a coordinated communist push. Third, the so-called fall of China in October 1949, when Mao Zedong's communists defeated Chiang Kai-shek's Nationalists, was widely viewed in the United States as a major setback. The 1950 National Security Council Report 68, NSC 68, called for a major military buildup to contain communism worldwide.

Fourth, the United Nations Security Council passed Resolution 82 on June 25, 1950 condemning the North Korean invasion and Resolution 83 on June 27 calling on members to repel it. The Soviet Union was boycotting the Security Council at the time over the seating of communist China, which allowed the resolution to pass without a Soviet veto.

President Harry Truman did not seek a formal congressional declaration of war but described the deployment as a UN police action under American command. General Douglas MacArthur led the United Nations Command, which eventually included troops from 15 nations. American forces ultimately bore most of the foreign casualties, with about 36,000 dead and more than 100,000 wounded.

The intervention preserved an independent South Korea, which became a wealthy democracy, and confirmed that the United States would back its containment doctrine with military force. The Korean War set the pattern for later Cold War interventions and foreshadowed the much larger conflict in Vietnam.