What caused the United States to send money and aid to South Vietnam?

Initially, it was a political strategic measure to prevent the spread of communism in Southeast Asia per our government's belief in the "Domino Theory". Where, if believed, one country after another would embrace communism until the entire region succumbed to the "Red Menace". It continued throughout even the war rather than admit we were wrong in intervening in what was really a civil war, even to the point of supporting a corrupt puppet government in RSVN. The Domino Theory of course was disproven, much to our government's chagrin.