Domino theory

. A political theory current in the 1960's, according to which the conversion of one country in South Asia to communism will start a sequential process causing all Asian countries to convert to Communism. The apparent assumption was that an Asian country politically aligned with the West was as politically unstable as a domino standing on edge. It was used by some as a justification for American involvement in the Vietnam war, 1964-1972. [ PJC ]