US President Harry S. Truman ordered air and naval forces to assist South Korea against an invasion by North Korea.
Towards the end of the Second World War in 1945 the ‘big three’ powers of the USA, the USSR and Great Britain met at the Yalta Conference. As part of a wide-ranging series of agreements, Korea was divided along the 38th parallel with a Soviet occupied zone in the north and Americans in the south.
In May 1948 the north, led by Kim Il-sung, declared itself the communist Korean Democratic People’s Republic. The democratic Republic of Korea soon followed in the south, and within a year both the USSR and the USA had withdrawn the majority of their troops. Meanwhile, the two Korean nations claimed the right to rule the other.
In the early morning of 25 June, North Korean troops crossed the 38th parallel and invaded South Korea. The action was condemned later that afternoon by the United Nations Security Council, who issued UN Security Council Resolution 82 which called for the North to immediately draw back to the 38th parallel. Two days later, on 27 June, the Security Council approved Resolution 83 which recommended providing military assistance to South Korea. The USSR could have been expected to veto both of these resolutions, but at the time was boycotting the Security Council over its refusal to admit the newly-proclaimed People’s Republic of China.
Truman announced American intervention shortly after Resolution 83 to broad approval from both Congress and the public. Fighting continued until 27 July 1953 when an armistice was signed that established the Korean Demilitarized Zone along the 38th parallel. A peace treaty was never concluded.