The U.S kept a few troops in South Korea after the Japanese troops occupying it surrendered to the U.S., and when the U.S. had only about 500 troops occupying the area, the Communists of North Korea attacked the 38th Parallel (the border between North and South Korea). The U.S. supported South Korea for the same reasons it supported Nationalist China: the Truman Doctrine, containment and America's intense fear of communists.
