The correct answers to this question are that the United States and France had been allies for more than 150 years; and if Vietnam fell to Communism, its neighbors, Laos and Cambodia, might fall too.
The Vietnam War and all the previous revolutionary process (which includes the war between Vietnam and France) must be understood as part of the Cold War, which confronted communism and capitalism.
France was, together with Great Britain, the main ideological ally of the United States on the capitalist side. On the other hand, the relations with France had been of collaboration from the same War of Independence of the United States (with some exceptions, like the Quasi-War). The United States had been the main collaborator of France in the Second World War, which was an event that had strengthened this relationship between both countries.
In addition, the Cold War posed the threat of communism. This had spread in Eastern Europe and Asia, thanks to the influence of the Soviet Union and China. Each new revolution that began was a threat to the system defended by the United States. In this context, the rise of Vietnam as a defender of communism, coupled with the influence that this revolution could have in neighboring countries of Southeast Asia, caused the United States to support France in the conflict and subsequently, the American Armed Forces entered directly in combat.
