What was the reason for US involvement in Vietnam.?

Answer 1

The United States sought to prevent the expansion of Communism in Southeast Asia and therefore tried to support right-wing dictatorships throughout Southeast Asia, like South Vietnam, the Kingdom of Cambodia, the Kingdom of Laos, and the Kingdom of Thailand, against Communist North Vietnam.

Answer 2

to stop the domino theory of when vietnam north and south were fighting over communism and capitalism. The US thought that if communism won that the domio theory would kick into play of where all neighboring countrys fall to current ruling near by. So we stepped into try to prevent something and another way of saying trying to make up the wrong in WWII by not reacting fast enough.