Why did the US support Vietnam?

Answer 1

Its government was anti-Communist because of the USA's "containment" policy, which had dictated US actions vis-a-vis the Communists since the late 40's. This policy held that Communist domination of territories must be contained within the areas where it was dominant back then, and that any Communist effort to expand must be repulsed by the US, by force if necessary. It had been the policy behind the USA's (successful) intervention in Korea a few years before.

Another American theory also was behind the USA's involvement: the "domino" theory, which held that if one Asian country would fall to Communism, all other Asian countries would successively follow like so many dominoes. As it was to turn out after the subsequent failure of the US involvement in Vietnam, the theory was wrong.