The Korean War (25 June 1950 – 27 July 1953) was a war between the Republic of Korea (supported by the United Nations) and the Democratic People's Republic of Korea (supported by the People's Republic of China, with military and material aid from the Soviet Union). The war was a result of the physical division of Korea by an agreement of the victorious Allies at the conclusion of the Pacific War at the end of World War II. The Korean peninsula was ruled by Japan from 1910 until the end of World War II. Following the surrender of Japan in 1945, American administrators divided the peninsula along the 38th parallel, with United States troops occupying the southern part and Soviet troops occupying the northern part.

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 capitalist one. The 38th parallel increasingly became a political border between the two Korean states. Although reunification negotiations continued in the months preceding the war, tension intensified. 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. It was the first significant armed conflict of the Cold War.