The domino theory was a theory during the 1950s to 1980s, promoted at times by the government of the United States, which speculated that if one land in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.
The domino effect suggests 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 domino theory was used by successive United States administrations during the Cold War to clarify the need for American intervention around the world.