When the french left Vietnam the US stepped in to?

Madness, sheer madness. The Domino Theory held that if one country fell to Communism, then the next would, and so on. This is bad because a) Communism is not very nice and b) Communist countries are not a market for US goods. Therefore the US must protect South Vietnam from the Commies in the North. However, as the French had already proved, this was impossible.