This was the first armed conflict of the Cold War. South Korea and United Nations (primarily U.S.) military forces opposed North Korea and its allies, China and the Soviet Union. Korea, partitioned at the 38th parallel after World War II, was under Soviet domination in the north. The U.S. was influential in the south. The eventual goal of uniting the two Koreas with free elections never materialized.

In 1950, North Korean forces invaded and almost completely conquered the outnumbered and outgunned South Korean and U.S. troops who clung to a small area called the Pusan Peninsula. However, Allied fortunes dramatically reversed following a daring U.S. landing at Incheon (formerly Inchon). U.N. troops regained the offensive, routing the overextended enemy and driving them back to North Korea. After U.N. troops conquered more of North Korea, its neighbor, Communist China, joined the war with a huge army in October 1950. Chinese and North Korean troops pushed U.N. troops back across the 38th parallel, and the military situation remained a bitterly fought stalemate until a cease fire was established. Korea became divided again at the 38th parallel with a demilitarized zone, as it remains today.