Respuesta :
The belief that communism would spread in Southeast Asia is the main reason for U.S. involvement in Vietnam.
Why did the US get involved in Vietnam?
China was the communist at the time of 1949, and had major control in the hands of North Vietnam. The USA got frightened that the communist of china will be going to spread and capture the South Vietnam and then the rest of Asia.
Thus, option D is correct.
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