Domino Theory
The Cold War belief that one country falling to communism would topple its neighbors in sequence, which justified American wars in Korea and Vietnam.
On April 7, 1954, with the French garrison at Dien Bien Phu about to fall, a reporter asked President Eisenhower why Indochina mattered to Americans. Eisenhower answered with an image: you have a row of dominoes set up, you knock over the first one, and what will happen to the last one is the certainty that it will go over very quickly. He spent fewer than 150 words on the point and mentioned the region's tin, tungsten, and rubber in the same breath. The metaphor outlived every argument around it and became the organizing assumption of American policy in Asia for the next twenty years.
The logic
The domino theory was an extension of containment into a claim about mechanism. Containment held that Soviet expansion had to be met at every point. The domino theory explained why even a marginal point mattered: because loss is contagious. A communist victory anywhere supplies its neighbors with a sanctuary, a supply line, a trained cadre, and above all a demonstration that the winning side is winning. Local elites read the trend and hedge. Insurgents recruit. Allies elsewhere begin to doubt the value of an American guarantee, and the credibility that holds a global alliance system together is a single indivisible thing.
Behind it was a specific historical memory. The men making these decisions had watched Czechoslovakia in 1948, the Berlin blockade in the same year, and the Chinese revolution in 1949, and they had learned from the 1930s that concessions to an expansionist power invite more. It was also politically compulsory. The domestic recriminations over who lost China had ended careers, and no American president after 1949 wanted to preside over a second such loss.
Where it applied and where it broke
The theory looked strongest in Korea, where an invasion was actually launched across a border by a communist state, and where the American commitment did hold the line. It looked weakest almost everywhere that mattered afterward, for three reasons.
First, it treated communist states as interchangeable pieces pushed by a single hand. The Sino-Soviet split was public by 1960, Yugoslavia had broken with Moscow in 1948, and Vietnamese communists had been fighting Chinese influence for a thousand years. Vietnam and China went to war in 1979. Dominoes do not fight each other.
Second, it discounted nationalism. Ho Chi Minh's movement drew its strength from being the vehicle of Vietnamese independence against the French and then the Americans, not from being a Soviet franchise. Fighting it as a domino meant fighting the strongest political force in the country while sponsoring governments that lacked it.
Third, the prediction failed the test. Saigon fell in April 1975. Laos and Cambodia fell the same year, but they had been fighting the same war and were arguably one domino, not three. Then it stopped. Thailand, Malaysia, Indonesia, and the Philippines did not go communist. Indonesia, the country whose loss Eisenhower and Kennedy feared most, had swung violently the other way in 1965. Within a few years of American withdrawal, Southeast Asia was consolidating around ASEAN and starting the fastest sustained economic growth in history.
The echo
Defenders of the theory make a serious counterargument: the American commitment in Vietnam bought a decade in which the region's shakier states stabilized, and the dominoes did not fall partly because someone spent enormous resources holding them up. That claim cannot be tested, which is exactly the problem with the theory as a guide to policy. It made every local conflict a test of global credibility while offering no way to check whether the mechanism was real in any particular case.
The reasoning did not retire with the Cold War. Every argument that a defeat in one place will cascade, that a red line unenforced anywhere weakens deterrence everywhere, that abandoning one ally teaches all the others to hedge, is a domino argument. Sometimes it is right. The lesson of Vietnam is that it is a hypothesis about a specific place, and that treating it as a law of physics is how a great power ends up fighting a war it does not need in a country it does not understand.