I should have mentioned the most important factor was the heroic struggle and sacrifice of the Vietnamese people, which did inflict many casualties on the US.

But the massive demonstrations in the US and around the world helped change the consciousness of millions, which ultimately even affected the thinking of the troops fighting in Vietnam. Many times the representatives of the NLF and DRV recognized the vital role of the US antiwar movement.

Unfortunately, both the USSR and China did not do much to help Vietnam, and Nixon was able to play them off pretty well.