The Domino Theory, first advanced during Harry S. Truman’s presidency, proposed that if a country came under the influence of Communists there would follow a “domino effect” that would spread communism to other nearby countries. This theory drove U.S. foreign policy in the period between the end of WWII and the collapse of the Soviet Union, known as the 'Cold War'. The Domino Theory was a phrase that became popular during the administration of U.S. President Dwight D. Eisenhower. According to this theory, once nations such as Vietnam fell to communism, other countries of Asia would fall like a row of standing dominoes, one after the other, each knocking the next over.
In the 1950s and the Vietnam War
- "The situation in that area, as we found it, was that it was subject to the so-called 'domino theory.'" "You mean that if one went, another would go?" "We are trying to change it so that would not be the case.... You generally have a whole series of countries that can be picked up one by one. That is the whole theory of the North Atlantic Treaty. When the nations come together, then the so-called 'domino theory' ceases to apply."
President John F. Kennedy endorsed the Domino Theory.  During the Vietnam War, the phrase was heard often. Supporters of the war gave the theory as a reason for staying in the war. The actual phrase "domino theory" though, was usually used dismissively by opponents of the war. They thought this theory was an exaggeration. A New York Times columnist mocked it in these words:
- ...two of the favorite propositions of its supporters have been particularly hard for opponents to accept. One is the so-called "domino theory"—that if South Vietnam fell to the Communists, other nations in the area would topple like dominoes until the United States was thrown back to Pearl Harbor, or perhaps the West Coast.
In the end, South Vietnam and Cambodia did fall to the Communists, and the regime of Pol Pot conducted a genocidal campaign in Cambodia.
In recent years
Before the 2003 Iraq war, neoconservative theorists suggested that a liberated Iraq could be a vanguard of democracy. A democratic Iraq would lead to the fall of authoritarian rulers in neighboring states, and the spread of democracy throughout the Middle East. This has sometimes been called the "democratic domino theory" or the "reverse domino theory." Thus far, it doesn't appear to be working.
- The Dulles' Statements on Indo-China, The New York Times, May 12, 1954, p. 6
- President Kennedy's View of the "Domino Theory,", News Conference, April 24, 1963. Source: The Pentagon Papers, Gravel Edition, Volume 2, pp. 818-819.
- Wicker, Tom (1967), "Southeast Asia: Dominoes and Master Plan," The New York Times, February 12, 1967, p. 180