domino theory, the notion that if one country becomes Communist, other nations in the region will probably follow, like dominoes falling in a line. The analogy, first applied (1954) to Southeast Asia by President Dwight Eisenhower, was adopted in the 1960s by supporters of the U.S. role in the Vietnam War. The theory was revived in the 1980s to characterize the threat perceived from leftist unrest in Central America.

or domino effect

Doctrine of U.S. foreign policy during the Cold War, according to which the fall of a noncommunist state to communism would precipitate the fall of other neighbouring noncommunist states. The theory was first enunciated by Pres. Harry Truman, who used it to justify sending U.S. military aid to Greece and Turkey in the late 1940s. Dwight D. Eisenhower, John F. Kennedy, and Lyndon B. Johnson invoked it to justify U.S. military involvement in Southeast Asia, especially the prosecution of the Vietnam War.

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