When America lost the Vietnam War, the domino theory was proven wrong.
In the initial stages of the Vietnam War, American policymakers thought that if South Vietnam fell to the Communism of North Vietnam, it would be a victory for the Communist (Soviet) side. However, when South Vietnam did finally fall to the North, it was evident that there was no longer a Communist "side", per se. Vietnam (united under Communism) wasn't interested in being a satellite state of the USSR, and even fell to disputing borders with China.