Good questions, both. I'll take the first one first.
Each of us has a conscience, an inhibiting sense of what is prudent. I argue for the existence of an "American conscience," or a collective conscience for the group.
Individual conscience speaks with votes, and those elected act for us as our collective conscience. The beauty of Democracy is that we can peacefully agree to put power into the hands of elected leaders who then take action for the Nation consistent with that conscience. Its a collective wisdom, and our Founding Fathers and Mothers believed it was the best way.
Few would dispute that nations, like individuals, can always be counted on to act in their own self-interest, and to inhibit actions which are not prudent for the nation.
Richard Nixon campaigned in '68 on a promise to end the war. Voters liked that. He was elected, and re-elected, and finally ended the war in 1973. Nixon, who is recognized today in many circles for his astute foreign policy, got the US out of Vietnam because, I must assume, he believed it was in our own collective self-interest.
The deaths of "over one million [Vietnamese]," (a figure I'll accept for the purposes of argument), is tragic. But I'm sure that Nixon, listening to the conscience of the Nation, realized that American blood is dearer, and was better shed somewhere else.
Why would Nixon, or any other head of a democracy in his position, allow a couse of action merely because he is swayed by some faction, i.e., liberals, when the best course of action lies elsewhere? He might get charged with treason. No, Nixon knew what he was doing was in the Nation's best interest.
For the second question, regarding where the war was lost, my answer has two parts. First, all wars must win on the political front. It is a constant war of words at home. That is what Democracy is all about. So I guess its fair to say it was lost there.
Second, the hidden premise of the question is that Vietnam was winnable. I think it was, but winning was only half the equation. There is always a cost to the nation in these wars, and the collection agency is very cruel.