I really don't think the US gave a hoot about South Vietnam. The US had attempted to isolate itself from world affairs, afraid of entangling alliances, for the past 60 years, and only got involved when they could stop the spread of Communitsm. What the Vietnam war about was containing Communism, not supporting South Vietnam; that just happened to be what they thought was the right way to go about it. I'm willing to bet that if there were just two groups, say, the equivalent of Republicans and Democrats (perhaps not the best example, but, not being a historian or politician, I cannot think of any other) fighting and killing one another, the US would shake a disapproving finger and then turn a blind eye. And the US was indeed the aggressor in the war. The North Vietnamese were probably not so crazy as to actively go up in arms against one of the world's two superpowers, even with support from the Soviet Union.