After 50 years I have come to the belief that we were never meant to win in Viet Nam.
Viet Nam, like Korea before it, were a way to fight the Soviet Union without going nuclear. It was a way to bleed the Soviets (they and the Chinese were providing the material to fight the war).
By the end of 1966 we had the man power and the air superiority to overwhelm the Viet Cong and the North Viet Names Army. We pissed away our advantage.
Over the past few days I have been reading newspaper articles that covered Nov 1965 to Nov 1966 (the time I was there).
I realized every operation we made was in the newspaper within a day of the operation beginning, along with the goals, and the means. The enemy did not need any spies to figure out what we were doing, the media was telling them, sometimes with maps!
Not only that, they were telling them how effective or not effective we were.
The enemy knew where we were, what we were doing, and more important where we were not. No one can win a war if the enemy knows that much information about you.
As long as you let the enemy have a King's X line you can never win anything. 38th Parallel comes to mind.