The Korean War (25 June 1950 - armistice signed 27 July 1953) was a military conflict between the Republic of Korea, supported by the United Nations, and North Korea, supported by the People's Republic of China (PRC), with military 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, and the North established a Communist government. 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.

The United Nations, particularly the United States, came to the aid of South Korea in repelling the invasion. A rapid UN counter-offensive drove the North Koreans past the 38th Parallel and almost to the Yalu River, and the People's Republic of China (PRC) entered the war on the side of the North. The Chinese launched a counter-offensive that pushed the United Nations forces back across the 38th Parallel.

The Soviet Union materially aided the North Korean and Chinese armies. In 1953, the war ceased with an armistice that restored the border between the Koreas near the 38th Parallel and created the Korean Demilitarized Zone (DMZ), a 2.5-mile (4.0 km) wide buffer zone between the two Koreas. Minor outbreaks of fighting continue to the present day.