The war

Phase 1: North Korean invasion (June-September 1950)

On June 25, 1950, North Korean forces invaded South Korea with Soviet equipment and Chinese acquiescence (probably not Soviet direct involvement, though Stalin had approved). South Korean and small American forces were nearly pushed off the Korean peninsula, holding only a perimeter around the port of Pusan.

Phase 2: UN counterattack (September-November 1950)

The United Nations Security Council (Soviet boycott meant the USSR could not veto) authorized military action to repel the invasion. U.S. General Douglas MacArthur led a brilliant amphibious landing at Inchon behind the North Korean lines (September 1950). North Korean forces collapsed. UN forces drove north past the 38th parallel and approached the Yalu River on the Chinese border.

Phase 3: Chinese intervention (November 1950 onward)

Communist China, fearing American forces on its border, intervened with hundreds of thousands of "volunteers" in late November 1950. Chinese forces drove UN forces back south of the 38th parallel. By spring 1951 the front had stabilized roughly along the original dividing line. MacArthur publicly called for expanding the war to China, including the use of nuclear weapons. President Truman fired MacArthur for insubordination in April 1951.