The Domino Theory was the American idea that a communist takeover in one country would quickly lead to communist revolutions in neighboring states, each falling like dominos until it reached the United States.
The Domino Theory was the driving force of American foreign policy during the Cold War. American intervention in Korea, Vietnam, and Afghanistan could be attributed as attempts to prevent the Domino Theory by preventing communist takeover at its source. The strategy also increased tensions between the United States and Soviet Union as American forces constantly attempted to undermine the Soviets.