Which of these best describes the “domino theory,” which was used as a justification for US involvement in Vietnam?

The domino theory held that if one country became Communist, nearby countries might follow, like a row of falling dominoes. This idea was used to justify US involvement in Vietnam as a way to prevent Communism from spreading through Southeast Asia. So the best answer is “If a country falls to Communism, then its neighbors are also likely to do so.”

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