At the end of World War II, Korea was divided at the 38th parallel into Soviet (North Korean) and U.S. (South Korean) zones of occupation. In 1948 rival governments were established: The Republic of Korea was proclaimed in the South and the People's Democratic Republic of Korea in the North.
Relations between them became increasingly strained, and on June 25, 1950, North Korean forces invaded South Korea. The United Nations quickly condemned the invasion as an act of aggression, demanded the withdrawal of North Korean troops from the South, and called upon its members to aid South Korea. On June 27, U.S. President Truman authorized the use of American land, sea, and air forces in Korea.
Casualties in the war were heavy. U.S. losses were placed at over 54,000 dead and 103,000 wounded, while Chinese and Korean casualties were each at least 10 times as high. Korean forces on both sides executed many alleged civilian enemy sympathizers, especially in the early months of the war.
Excerpted from: Korean war. Retrieved 02/18, 2014, from http://www.infoplease.com/encyclopedia/history/korean-war.html