What was America worried would happen to all of Southeast Asia if Vietnam fell to the

communist?

The domino theory was a Cold War policy that suggested a communist government in one nation would quickly lead to communist takeovers in neighboring states, each falling like a perfectly aligned row of dominos. In Southeast Asia, the U. S. government used the now-discredited domino theory to justify its involvement in the Vietnam War and its support for a non-communist dictator in South Vietnam. In fact, the American failure to prevent a communist victory in Vietnam had much less of an impact than had been assumed by proponents of the domino theory. With the exception of Laos and Cambodia, communism failed to spread throughout Southeast Asia.