After North Korea attacked South Korea, America felt that North Korea broke world peace by attacking South Korea. (North Korea was controlled by the Soviet Union, and South Korea was controlled by the United States) Truman didn't want the spread of communism. So, he went to the UN to ask them to send an army out to Korea.
UN's Action
The UN agreed to send troops out to Korea (China and the Soviet Union wasn't there to oppose). In 1950, UN landed in North Korea. After the success of the landing, the UN sent half of the soldiers to South Korea. General MacArthur was vocal about also attacking China.
Douglass MacArthur
Douglass MacArthur was a American General, most popular for his command of allied forces in the Pacific during World War 2. In 1945, he was appointed as supreme Allied Commander, by Truman. He was also in charge of the official surrender of Japan. During the Korean Conflict, he was vocal about his belief that that the war should be spread to China. Truman wanted him to keep his opinions to himself . In 1951, Truman relieved MacArthur of his command.