What was the most common argument used by the US presidents of the 1960s and 1970s favoring military involvement in Vietnam?

U.S. presidents most often justified involvement in Vietnam through the policy of containment: they argued that the United States had to prevent communism from spreading to other countries. This reasoning was often described as the domino theory, which predicted that one country becoming communist could lead others to follow. So the correct choice is C, stopping the potential spread of communism.

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