Ouch! I always have a problem with that perception....true, the communist north defeated the forces of RVN, but not until a couple of years after the last combat troops left Viet Nam.
When JFK first ramped up the advisors to RVN, the purpose was to assist RVN in it's effort to fight off a communist take over. When it appeared, more was needed, LBJ ordered the escalation of American involvement. (both Democrats....hmmmm.)
I am purposely leaving out the political considerations that led to these decisions; too many to discuss in this post.
In 1968 after TET the communists were so weak, the war was winding down, but a bizarre chain of events involving many puppets on this stage we call life came into play or became more prominent.
The decision to stop the bombing in the north.
The decision to allow the build up of material and men just across "neutral borders", the efforts of SOG notwithstanding.
McNamara's often strange advice on the way to conduct the war.
LBJ's decision to not run again.
The election of RM Nixon and the perception/belief of the American public that he must/should withdraw the troops.
Kissinger's selection.
The idiotic misinformed statements of Uncle Wally about the unwinnable war.
On and on.
A lot more came into play in the decision to leave RVN and let them carry the fight, but I believe, America did not lose that war, the Vietnamese did, and the only egg on our face was our lack of re-intervention in 1975. It created a world wide suspicion of American resolve, for which we are still paying the piper.
Sorry. I should have been more accurate. I see your point of view, and agree the game was up when the RVN gave up on itself. Regardless, the US didn't 'win'in Viet Nam, in the conventional sense, anyway. Except now, in hindsight, we can see that the so-called 'divisive' and 'aggressive' policies of the United States saved much of Southeast Asia and Oceania from communist domination and destruction. With all the similarities to Viet Nam the RATS have been trying to draw with Iraq, I'm surprised no one has mentioned this one: it was the right thing to do; it was a huge sacrifice; The United States of America was proved right in the long run.