Here is an article by the aforementioned Lewis Sorley. I’m hardly an expert on the Vietnam war, but I pass it along. Here’s an excerpt:

In the later years of American involvement, during which U.S. ground forces were progressively being withdrawn, priority for issue of M-16 rifles was given to the long-neglected South Vietnamese Territorial Forces, who provided the “hold” in clear and hold. As those forces established control over more and more territory, large numbers of VC “rallied” to the allied side. This reached a peak of 47,000 in 1969, with another 32,000 crossing over in 1970. Given the authorized 8,689-soldier strength of a North Vietnamese Army division, that amounted to enemy losses by defection equivalent to about nine divisions in those two years alone.
There came a point at which the war was as good as won. The fighting wasn’t over, but the war was won. The reason it was won was that the South Vietnamese had achieved the capacity, with promised American support, to maintain their independence and freedom of action. This was a South Vietnamese achievement.