The domino theory was a 20th Century foreign policy theory that speculated if one land in a region came under the influence of Communists, then more would follow in a domino effect. The domino effect indicates that some change, small in itself, will cause a similar change nearby, which then will cause another similar change, and so on in linear sequence, by analogy to a falling row of dominoes standing on end.

The theory was used by many United States leaders during the Cold War to justify U.S. intervention in the Vietnam War. The domino theory was applied by President Dwight D. Eisenhower and his top advisers in 1954 to describe the prospects of communist expansion in Asia if Indochina were to fall. Eisenhower argued that all of southeast Asia could fall. The theory's ultimate validity remained mixed, and debatable. After the U.S. left Vietnam, the North took over the South, and Cambodia and Laos had also turned to Communism, although Cambodia is no longer a communist state. This limited spread of Communism in Indochina provides ammunition for opponents of the theory, but both sides argue that the historical record overall supports their position.

In the 1980s, the domino theory was used again to justify the Reagan administration's interventions in Central America and the Caribbean region.

From its first conception, many have disputed central assumptions of the domino theory, for instance by arguing that Communist States lacked the tradition of cooperation the theory assumes (eg Cambodia attacked Vietnam, to which Vietnam responded by overthrowing the Khmer Rouge government). Supporters however have continued to argue it was a sensible policy in the context of the times.

From the website of http://en.wikipedia.org/wiki/Domino_theory