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.[29]
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 38th Parallel increasingly became a political border between the two Koreas. 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.[30] It was the first significant armed conflict of the Cold War.[31]
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.[30] 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