The Korean War (25 June 1950 - 27 July 1953) was a war between the Republic of Korea (South Korea), supported by the United Nations, and the Democratic People's Republic of Korea (North Korea), at one time supported by China and the Soviet Union. The Korean Peninsula was ruled by the Empire of Japan from 1910 until the end of World War II. Following the surrender of the Empire of Japan in September 1945, American administrators divided the peninsula along the 38th parallel, with U.S. military forces occupying the southern half and Soviet military forces occupying the northern half.

The failure to hold free elections throughout the Korean Peninsula in 1948 deepened the division between the two sides; the North established a communist government, while the South established a right-wing government. The 38th parallel increasingly became a political border between the two Korean states. Cross-border skirmishes and raids at the 38th parallel persisted. The situation escalated into open warfare when North Korean forces invaded South Korea on 25 June 1950. In 1950, the Soviet Union boycotted the United Nations Security Council. In the absence of a veto from the Soviet Union, the United States and other countries passed a Security Council resolution authorizing military intervention in Korea.

Before the invasion, Korea was not included in the strategic Asian Defense Perimeter outlined by Secretary of State Acheson. Military strategists were more concerned with the security of Europe against the Soviet Union than East Asia. This idiotcy gave the North Korean carte blanche to invade as we didn't care about Korea.