Why did the United States send military aid to South Vietnam?

The United States sent aid to Southern Vietnam due to the ideological conflicts between South and North Vietnam. As the North Vietnamese were being aided by communist countries like China and the Soviet Union. This irritated the United States and did not want Vietnam to fall into Communism. They were also worried about the "Domino Effect" which meant once Vietnam became communist, so would Laos, Cambodia, Thailand, Myanmar, etc.