Sorry to say I find your assertion the the USA "could not" have won the war in Vietnam, in a word, silly.
Of course we could have won, we could have paved the place if we mobilized the nation. If we had simply occupied the Ia Drang that alone would have delivered us the central highlands, and according to Vietnamese military doctrine going back centuries, all of Vietnam. Of course that would have required mobilization, which in turn would have hampered LBJ's plans to socialize the USA.
Couldn't have won my arse, were it not for the emergence of a heavily propagandized "anti war" movement nurtured by the same collectivist totalitarian leftovers from the American Communist Party that we call "liberals" today our utter demolition of the NVA in the Tet offensive would have resulted in a negotiated peace.
It is also true that the craven dishonesty and monumental (and monumentally unjustified) ego of LBJ in his pursuit of self aggrandizement and his idiotic leftist politics were to blame for providing fetile soil for the totalitarian nazis of the American left to pursue their anti freedom agenda. LBJ is arguably the worst president in American history, on one hand hamstringing the military to allow his leftist "great society" programs, on the other, lying to the American public and creating an opening for the "anti war" cretins who continue to inflict their hateful damage upon American liberty to this day.
American GI's after Tet noted that the NVA were often captured whacked on heroin, evincing total demoralization which spoke volumes about the same sense of total defeat voiced by Giap from the top.
Then Cronkhite steps in and hands the North Vietmanese their new, and ultimately victorious strategy, convince the USA to do itself in using the twisted minds of collectivist totalitarian nazis right here at home.
We did not seek war in Vietnam, we never seek war. We fought to stem the tide of global communism and arguably the conflict in Vietnam was the beginning of the end of that vile movement. Sadly the Hanoi jane fondas of the world did facilitate our withdrawl from Vietnam, and while we did eventually emerge victorious from the cold war, over a million people died at the hands of communist mass murderers after we left, and that is the fruit of the successful domestic propaganda camapaign which the American left has been desperately seeking to avoid accepting responsibilty for ever since.
More than a million innocent civilians did not have to die in southeast Asia, but for LBJ's spectacular lack of character, integrity, and judgement, and the twisted politics of colectivist totalitarians that blight domestic politics to this day. Finally, the countless millions held in communist slavery ever since have the fondas of the world to thank for their bondage.
One who claims that we "couldn't" have won presumed knowledge impossible to own. Might be reaching far beyond credulity to avoid the truth.
We did win the struggle against global communism, we just didn't care to keep a million Vietnamese necks off their murder's blades.
After 199 posts to this thread you nailed it precisely!
Fact is that there never was a time, from Jan. 1966 - Dec. 1970, when the war couldn't have been definitively won, within 6 months, given the political WILL to do so!