Why was the United States concerned about Vietnam during the 1950s?

During the 1950s, the United States feared that if Vietnam became communist, communism might spread to nearby countries—a concern often called the domino theory. Laos and Cambodia were neighboring countries, so preventing communism from spreading there was a major reason for U.S. concern. The United States did not primarily oppose French control at that point; it supported France’s effort to contain communism in Vietnam.

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