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The domino theory was a reason for war during the 1950s to 1980s, promoted at times by the government of the United States, that speculated that if one state in a region came under the influence of communism, then the surrounding countries would follow in a domino effect. The domino theory was used by successive United States administrations during the Cold War to clarify the need for American intervention around the world.
The primary evidence for the domino theory is the spread communist rule in three Southeast Asian countries in 1975, following the communist takeover of Vietnam: South Vietnam (by the Viet Cong), Laos (by the Pathet Lao), and Cambodia (by the Khmer Rouge). It can further be argued that before they finished taking Vietnam prior 1950s, the communist campaigns did not succeed in Southeast Asia. Note the Malayan Emergency, the Huk Rebellion in the Philippines, and the increasing involvement with Communists by Sukarno of Indonesia from the late 1950s until he was deposed in 1965. All of these were unsuccessful Communist attempts to take over Southeast Asian countries which stalled when communist forces were still focused in Vietnam.
Finally, you have broader considerations that might follow what you would call the "falling domino" principle. 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. So you could have a beginning of a disintegration that would have the