Faced with a military setback and diplomatic isolation, Le Duc Tho, Hanoi's principal negotiator, abandoned Hanoi's 1969 terms in October 1972. He accepted conditions publicly put forward by Nixon in January 1972 -- and decried as unachievable in the U.S. domestic debate. The terms of the resulting Paris peace agreement were an unconditional cease-fire and release of prisoners; continuation of the existing South Vietnamese government; continued U.S. economic and military help for it; no further infiltration of North Vietnamese forces; withdrawal of the remaining U.S. forces; and withdrawal of North Vietnamese forces from Laos and Cambodia. None of these terms were available in 1969.
The Nixon administration was convinced that it had achieved a decent opportunity for the people of South Vietnam to determine their own fate; that the Saigon government would be able to overcome ordinary violations of the agreement with its own forces; that the U.S. would assist against an all-out attack; and that, over time, the South Vietnamese government would be able to build a functioning society.
American disunity was a major element in dashing these hopes. Watergate fatally weakened the Nixon administration through its own mistakes, and the 1974 midterm congressional elections brought to power the most unforgiving of Nixon's opponents, who cut off aid so the agreement couldn't work as planned. The imperatives of domestic debate took precedence over geopolitical necessities.