domino theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A theory that one event will set off a train of similar events, especially.
• noun The theory, widely espoused during the Cold War, that if one nation comes under Communist control, then nearby nations will also come under Communist control.
• noun Any of various theories predicting that a sweeping political change in one nation will cause similar changes in nearby nations.

from the GNU version of the Collaborative International Dictionary of English.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun politics, historical The theory that, if South Vietnam fell to Communism, it would be followed by Cambodia, Laos, and additional South Asian countries.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the political theory that if one nation comes under communist control then neighboring nations will also come under communist control