The reason for us involvement in the Korean War?

However, the entry of the United States into the conflict signaled a reversal of policy toward Korea. Although it backed the government of Syngman Rhee, the United States had begun withdrawing its troops from South Korea in 1948. As late as January of 1950, Secretary of State Dean Acheson had implied that the Korea Peninsula lay outside the all-important "defense perimeter" of the United States, a statement that some took to mean that the United States would not defend the ROK from communist attack.

So why did the United States become involved in the Korean conflict? The decision to intervene in Korea grew out of the tense atmosphere that characterized Cold War politics. On the eve of the North Korean invasion, a number of events had made Truman anxious.

The Soviet Union exploded an atomic bomb in 1949, ending the United States' monopoly on the weapon. In Europe, Soviet intervention in Greece and Turkey had given rise to the Truman Doctrine and the Marshall Plan, which funneled aid to war-torn Europe in the hopes of warding off communist political victories. In early 1950, President Truman directed the National Security Council (NSC) to conduct an analysis of Soviet and American military capabilities.

In its report, known as "NSC 68," the Council recommended heavy increases in military funding to help contain the Soviets.