In 1950, Truman as part of his policy of Containment sent troops to South Korea to assist the South Korean army in defeating the North Korean Army. The goal was to push the Northern army back to the 38th Parallel. General MacArthur was able to get troops behind the North Koreans with an amphibious assault which resulted in the North pulling their men back. With this success MacArthur decided to try to invade the North but was stopped when the Chinese entered the war and halted the American advance. America, working as part of NATO had to agree to keep the nation split along the 38th, a truce that is still active today.
Historically Speaking 