Vietnam, Korea, and Cambodia were huge american policy interventions to stop communism. Why didn't the US similarly launch an invansion into China? The size?
Harry Truman authorized billions to go to Chai-shek's Nationalists, but it didn't turn the tide (1949). That's probably why starting with Korea in 1950, the United States began to send troops. China was the first of the dominoes to fall. (See Domino Theory)