The Vietnam War was lost by the West because it was controlled entirely by politicians, not by competent high-ranking military personnel.
It was lost because the most basic tenet of War was not carried out - that of fighting for, gaining control of, and holding ground, once that ground was won.
In Vietnam, no ground areas were ever captured and securely held - thus the NVA and VC were largely free to move around at will. Stopping enemy movement within your held territory is crucial to winning a war.
The Vietnam War was lost because America supported a totally corrupt, base, ruthless and decadent leadership group in the South. These people didn't even have the support of the largest majority of the South Vietnamese.
America failed to address one of the major divisive features in the South - the internal hatreds and battles between the Catholics and the Buddhists.
If this had been properly addressed, the South would not have produced so many guerrillas willing to support the North.
As far as "losing militarily" goes, I have this point of fact buoying me for the Australian efforts in Phuoc Tuy province.
Almost adjoining the Australians Nui Dat base was the NVA D445 Battalion. The Australians doggedly pursued D445 and made their life pure misery.
An NVA Battalion in 1965 had a full-strength complement of 555 men. I knew we had reduced the numbers in D445 substantially - but I did not know how much we had done that, until I read an interview with a former NVA Colonel in the 1990's.
In that article, the Colonel stated that the Australians were such ferocious and competent fighters, that D445 Battalion had been reduced to a complement of just 3 men, by 1971.
That speaks volumes for me, and shows how close we actually came to winning the Vietnam War.
It was the politicians who failed to ensure that the war gains made, were capitalised on, and not thrown away, that led to the West's loss in Vietnam.