The Start of the Korean War
In 1949, China became communist and a disagreement over which of the four (five before China left) permanent members (GB, France, USA and Russia) should occupy China’s seat broke out. Russia was very annoyed by this disagreement and subsequently left the Security Council. It thought, quite rightly, that the seat should be occupied by the communists under Mao Tse-Tung although the west said that the seat should be occupied by the nationalists whose leader was Chiang Kai-Shek.
Events of the Korean War
The war started on 25th June 1950 when North Korea invaded the south. Both sides claimed the whole of Korea and Russian tanks and aircraft supported the invasion by the north. On 26th June, the UN ordered the north to withdraw and the next day asked member countries to send aid to South Korea.
Aid arrived just as North Korea was threatening Pusan in the south (in August). General MacArthur ordered the successful Inchon Landings on 15th September of that year and soon the UN captures Seoul and reached the 38th parallel. The north then retreated back into North Korea.
A turn around came when the U.N. troops advanced towards the Yalu River. In October 200,000 Chinese "volunteers" forced the UN back into the South but although it looked as if the communists would win the war, UN counter attacks drove them back to roughly the 38th parallel.
General MacArthur was replaced with General Ridgeway after MacArthur openly talked openly about using nuclear weapons and invading China. He was sacked by President Truman who also feared that MacArthur would invade China.