The Domino Theory - Origins of the theory
To the extent that there was a historical foundation for the theory, it lay in the origins of World War II. Late in 1938 at the Munich Conference, Britain and France had allowed Adolf Hitler to take the Sudetenland from Czechoslovakia. Many people believed in retrospect that Hitler would have been relatively easy to stop at that time, if the democracies had stood firm in defense of Czechoslovakia. There is no way to tell whether this belief was correct, but at least it was not obviously foolish. However, the opportunity to try to stop Hitler before he became too strong was not taken. Within less than two years after Munich, Hitler partitioned what had remained of Czechoslovakia; invaded and partitioned Poland; overran Denmark, Norway, Luxembourg, the Netherlands, Belgium, and France; and started the Battle of Britain.
The end of World War II eliminated Hitler and his government, but Joseph Stalin remained. Stalin was a ruthless and extraordinarily brutal dictator, a mass murderer on a huge scale. He had emerged from World War II controlling not only the Soviet Union but also much of East-Central Europe. Stalin seemed obviously similar to Hitler, and some of the differences between the two— that Stalin ruled a larger area with a larger population and more industry than had Hitler, and that control of the international communist movement gave Stalin a worldwide influence—made Stalin seem a greater menace than Hitler had been. In 1949 Stalin got the atomic bomb, and the conclusion of the Chinese civil war brought the largest country in the world under communist rule, which many in the United States assumed to mean Stalin's rule.
Even before 1949 some people had begun to worry that any further territorial gains by Stalin or communism—little distinction was made between the two—could trigger a cascade effect, like the rapid sequence of Nazi conquests in the two years after Munich. Dean Acheson described in his memoirs the arguments he had used in February 1947 to persuade U.S. congressional leaders that they must support measures to prevent a Communist victory in Greece:
I knew we were met at Armageddon…. Likeapples in a barrel infected by one rotten one, the corruption of Greece would infect Iran and all to the east. It would also carry infection to Africa through Asia Minor and Egypt, and to Europe through Italy and France, already threatened by the strongest domestic communist parties in Western Europe.
The geographic logic was rather peculiar. For Greece to have gone communist would not have opened up a single new country to communist contagion, assuming that communism spread by contagion like rot from one apple to another in a barrel, because the only non-communist country bordering on Greece was Turkey, and Turkey already bordered directly on the Soviet Union. The congressional leaders, however, seem to have found Acheson's case persuasive. It was also suggested that if West Berlin (a small and strategically valueless enclave in the middle of Soviet-controlled East Germany) were to be lost, then Western Europe as a whole would crumble and fall under Soviet domination.