The Korean War was a military conflict 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 and the Soviet Union. The war began on 25 June 1950 and an armistice was signed on 27 July 1953.
The war was a result of the political division of Korea by agreement of the victorious Allies at the conclusion of the Pacific War. The Korean peninsula had been ruled by Japan prior to the end of the war; in 1945 following the surrender of Japan the peninsula was divided by American administrators 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 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 the North Korean forces invaded South Korea on June 25, 1950. It was the first significant armed conflict of the Cold War.
The United Nations, particularly the United States, came to the aid of the South Koreans in repelling the invasion. After early defeats at the hands of the North Korean military, a rapid UN counter-offensive repelled the North Koreans past the 38th Parallel and almost to the Yalu River, and the People's Republic of China (PRC) came to the aid of Communist North. With Communist China's entry into the conflict, the fighting took on a more dangerous tone. The Soviet Union materially aided North Korea and China, and the threat of a possibly nuclear world war eventually ceased with an armistice that restored the border between the Koreas at the 38th Parallel and created the Korean Demilitarized Zone, a 2.5-mile (4.0 km) wide buffer zone between the two Koreas. North Korea unilaterally withdrew from the armistice on May 27, 2009, thus returning to a de jure state of war.
During the war, both North and South Korea were sponsored by external powers, thus facilitating the war's metamorphosis from a civil war to a proxy war between powers involved in the larger Cold War.
From a military science perspective, the Korean War combined strategies and tactics of World War I and World War II — swift infantry attacks followed by air bombing raids. The initial mobile campaign transitioned to trench warfare, lasting from January 1951 until the 1953 border stalemate and armistice.