The Korean War was an armed conflict fought primarily on the Korean Peninsula from 25 June 1950 to 27 July 1953, between the Republic of Korea (South Korea) and its allies-principally the United States and other United Nations member states-and the Democratic People's Republic of Korea (North Korea), supported by the People's Republic of China and the Soviet Union. The war followed the post-World War II division of Korea along the 38th parallel and resulted in millions of military and civilian casualties without a formal peace treaty; the conflict ended in an armistice that established a demilitarized zone and left the peninsula divided. Interpretations of the war's origins, character, and historical significance remain contested; see korean-war-origins-debate.
Historians broadly agree on the war's chronology: North Korean forces crossed the 38th parallel on 25 June 1950; UN forces, authorized by UN Security Council Resolution 83, intervened under U.S. command led by General Douglas MacArthur; Chinese forces entered the war in October 1950 after UN forces approached the Yalu River; and the front stabilized near the original border by mid-1951, followed by two more years of attritional fighting and negotiation. The Korean Armistice Agreement was signed on 27 July 1953, but no peace treaty has been signed, leaving the two Koreas technically still at war. Debate continues over the war's origins, particularly the degree to which it was initiated independently by North Korea, encouraged by Soviet and Chinese leadership, or shaped by U.S. and Soviet Cold War strategy; see korean-war-origins-debate. Casualty estimates also vary significantly by source and methodology. 1)
For a fuller historical account, see korean-war-history.