Harry S. Truman was president in 1950 when the Korean war started. Truman committed the troops to fight with the soldiers from the United Nations due to the possibility that communist China and the Soviet Union may have encouraged North Korea's invasion into South Korea.
The surprise attack that occurred on June 25th, 1950, was massive and believed to be due to the growth of strength that was happening in the southern region despite armed uprisings in the 1940s that called for unification of the two parts of Korea. Once committed to supporting South Korea in its efforts to maintain its republic, Truman named General MacArthur to head the United Nations Command. The first few months of the battle were characterized by both sides advancing and retreating multiple times up and down the peninsula. In October of 1950, the United States government urged the United Nations to advance past the 38th parallel to try to unify Korea under one noncommunist government. China became involved because of the advancement, and once more the troops were in an exchange of advance and retreat. The Truman administration abandoned the plan to unify North and South Korea in 1951, instead opting to maintain the separate states and prevent a third world war with China and the Soviet Union.