the.com/domino theory

means A political theory holding that if one nation in a region adopts communism, neighboring nations will inevitably follow in a chain reaction, toppling like dominoes.

from Coined from the game of dominoes, where tipping one tile standing on end sets off a cascading chain reaction down the whole line. The phrase entered political language in the 1950s, famously used by U.S. President Eisenhower when discussing Southeast Asia, picturing nations lined up like tiles ready to fall one after another — an image that shaped decades of Cold War foreign policy and justified interventions, most notably in Vietnam.