The Endless Search for Reasons

You're on the right track: Vietnamese determination did play a large role in the war. For the sake of argument, take a look at strictly the military battles, who won, who lost, and then look at the political involvement in military planning, a job for which they are not suited. Before you protest too much, think back on how Hitler himself meddled into the affairs of military strategists to a point that he dictated virtually every aspect of the war, even though he had no experience or training to do so. Results are obvious. I think you will find that the US military, for all their faults and political constraints, won almost every major battle, including both Tet offensives. Not being able to press the attack contributed to the end defeat. I believe our political motives for even being there were seriously flawed anyway, giving way to a self-fulfilling prophecy.