The domino effect is the idea 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.
A well-known use of this metaphor was a justification for the Vietnam War, coined by Eisenhower: if Vietnam fell to communism, then Cambodia would be next, then Laos, and eventually all of Southeast Asia. It was also used by the USA during the Johnson administration to justify support for the overthrow of Indonesian president Sukarno in 1965.
See also: Butterfly effect, Snowball effect, Mathematical induction, Domino theory, Slippery slope