The US entered the Vietnam to end Communism or to prevent it from spreading like a plague. This is the most simple way to answer this question.
Vietnam was still considered French territory but at some point there was an insurgency, which was lead by the communists. There is a term that was used to describe the spread of communism and that was called the Domino Theory. The Domino Theory states in layman's term that if communism took over Vietnam it would then spread to other Asian nations like it had in Eastern Europe.
Hope this helps.
