Why did the anti war movement in the US during Vietnam prevent American forces from winning the war?

Returning GI's had left one war, and returned home to another one. The military draft (40,000 men a month) had fed the flames of protests and riots, and reached it's apex with the killing of four college students at Kent State University in Ohio on 04 May 1970 (memorialized by Crosby, Stills, and Nash, (and Young) with..."OHIO"). One of the best ways to lose a war, is by fighting one on TWO FRONTS; Fighting one war in Vietnam, and fighting another one on the home front; we had a divided nation. The Communists had used one of the classic rules of war: Divide and Conquer.