Just some thoughts.
We were in VietNam due to anti-Communist attitudes. That was the big bugaloo back then. Now it's Islamic extremists (not without reason!!!!!). I just find it interesting that then it was Communism we were afraid of and now it's Islamic extremists.
OTOH, no one knows what might have come about had we never gone into VietNam. Would the Communists of the time tested us elsewhere in worse ways? Would they have swept over SE Asia or elsewhere had we not been ready to fight. We may have lost in VietNam but, did we? It's a question that can never be answered for sure.