agreed, leaving the south to be slaughtered by the north is a horrible stain on our reputation.
Obviously we didn't achieve our goal of uniting the north and south, but we never really lost a battle. I mean, south vietnam fell 2 years after all us troops left the battlefield.
Was it worthwhile? I think it showed that we were willing to fight the spread of communism and may have helped put the seed doubt in other countries minds. I look at vietnam as more of a battle than a war....one battle in the cold war.
We can look at history and say "see, the domino effect was bullshit." I dont believe we can do that, because we tried very hard to stop it with korea, vietnam and other operation. How far communism would have penetrated without US intervetion is unknown. We know the outcome when there was US intervention: Communism died. So was it worthwhile? Yes. That doesn't mean i'm glad that some 2 million or whatever people died however. Vietnam is quickly becoming a very capitalistic country and will one day become a democracy. Who wins then? We do. Vietnam is already becoming a very important friend of the US's. Not that long ago we had a US naval ship in port in vietnam as a sign of friendship.
With all that said, we executed the war very poorly and could have done things a lot better to 1) help "win" the war and 2) save more lives.