I think Goldwater was dead right on that one. Vietnam never attacked America. We went in on the say-so of a small circle of Camelot Democrat "best and brightest", the Kennedy equivalent of Roosevelt's brain trust. Goldwater understood that our military power is to be reserved for times when the nation's existence is threatened - and that when this happens, we need to fight to win.
Vietnam, America's ally, was attacked. That's kind of the point of the war.
America helps its allies, especially when its allies are being attacked by proxy armies of international Communism.