The Korean War began when North Korean forces invaded South Korea in June 1950, crossing the 38th parallel border between the two countries. The invasion was backed by the Soviet Union, which had established North Korea as a communist state and supplied it with weapons and military advisors. The United States decided to intervene on behalf of South Korea through the United Nations to stop the invasion, seeing it as an act of Soviet aggression that breached postwar agreements. With UN authorization, US and other UN forces pushed the North Korean army back above the 38th parallel, but China then intervened on North Korea's side, leading to a prolonged war.