Once the theory had become a bone of contention, some defenders of the war began to moderate the theory so as to make it more reasonable; then, opponents of the war would not be able to ridicule it and say that its absurdity symbolized the absurdity of the war. When Richard Nixon defended the domino theory in 1970, he did not say (as he had five years earlier) that other nations would promptly fall to communism if South Vietnam fell; he said only that such an event would be "immensely discouraging" to the non-communist nations of Asia, and "ominously encouraging" to China and the Soviet Union.
The fall of South Vietnam in 1975 did not trigger the fall of the rest of Southeast Asia or of any long string of dominoes. Cambodia had already fallen before South Vietnam did. The fall of Laos was in a sense triggered by the fall of South Vietnam, but only because the Vietnamese communist leaders in Hanoi, who could have completed the communist victory in Laos long before, had been holding back because they feared a premature victory in Laos might complicate the struggle for South Vietnam. The United States did not have to fight a war on some line farther back to prevent the further spread of communism. Indochina under communist rule did not become a conduit for the application of Chinese pressure against Thailand. Instead, in the 1980s Thailand and China were allied against the communist governments in Indochina.
These events have not ended the arguments about the validity of the domino theory. Opponents of the theory say that it was proven false when South Vietnam fell without triggering the fall of most of Southeast Asia, and without the United States even having had to make any major effort to prevent the fall of the rest of Southeast Asia. Supporters of the theory say that it had been a correct description of the situation of the early 1960s. They say that the American defense of South Vietnam had provided a shield during the years when Thailand, Malaysia, and Indonesia had been vulnerable and China very aggressive. By the time this shield was removed in the 1970s, the Southeast Asian nations were stronger and China much mellowed. The most detailed form of the argument states that it was the firm American stand in Vietnam that gave anticommunist military officers the confidence to defeat the communists in Indonesia in 1965. The evidence for this argument, however, is very thin.
A cascade effect very much like that predicted by the domino theory swept communist regimes out, rather than into, power at the very end of the Cold War. In 1989 it became apparent that the Soviet Union under Mikhail Gorbachev would no longer intervene to preserve communist governments in Eastern Europe. This emboldened anticommunist forces and so discouraged the communist leaders that in one country after another they allowed communist rule to be overturned without a serious fight. Communism collapsed in most of Eastern Europe and in the Soviet Union itself between 1989 and 1991.