North Korean forces invaded South Korea on 25 June 1950

The situation escalated into open warfare when North Korean forces invaded South Korea on 25 June 1950.^[13] It was the first significant armed conflict of the Cold War.^[14] In 1950 the Soviet Union boycotted the United Nations security council, in protest at representation of China by the Kuomintang / Republic of China government, which had taken refuge in Taiwan following defeat in the Chinese Civil War. In the absence of a dissenting voice from the Soviet Union, who could have vetoed it, the United States and other countries passed a security council resolution authorizing military intervention in Korea.

http://en.wikipedia.org/wiki/Korean_War