Despite pledges to protect South Vietnam, former US President Richard Nixon privately vowed to "cut off the head" of its leader unless he backed peace with the communist North, tapes released showed.
The tapes appear to confirm charges by South Vietnam's late president, Nguyen Van Thieu, who tearfully accused the US of breaking its word to protect Saigon when the southern capital fell in 1975.
The National Archives released more than 150 hours of new tapes from Mr Nixon, who notoriously recorded his conversations. Mr Nixon is heard railing against the media and Congress for allegedly undercutting the war effort in Vietnam.
Hours before his second inauguration in January 1973, Mr Nixon telephoned top aide Henry Kissinger and urged him to press Mr Thieu to back the Paris Peace Accords which ended most US military involvement in Vietnam.
Mr Nixon asked Mr Kissinger to tell the South Vietnamese - truthfully or not - that the US Congress would cut off aid to the Saigon government unless it supported the accords.
"I don't know whether that threat goes far enough or not but I'd do any damn thing that is - or cut off his head if necessary," said Mr Nixon.
Mr Kissinger indicated it would be easy to strong-arm South Vietnam's foreign minister, Tran Van Lam, who was in the French capital for the talks.
"The foreign minister is an ass and he won't be able to do anything," said Mr Kissinger, then Mr Nixon's national security adviser, who sealed the agreement in Paris three days later with top diplomats from the two Vietnams.
But Mr Nixon met the foreign minister later that month at the White House and promised to do "everything that we can" to assist South Vietnam, according to another tape released on Tuesday.
"Your independence - your ability to keep the communists (inaudible), your ability to keep South Vietnam under your control, the opportunity to have an international agreement of their own choice - this for me is very important," added Mr Nixon.
"The main thing to remember: We know who our friends are," said Mr Nixon.
North Vietnamese troops seized Saigon in 1975, after Mr Nixon had resigned in the Watergate scandal.
Mr Thieu in a bitter farewell address called the US an "inhumane ally," saying the Nixon team forced him to sign the Paris Peace Accords under false promises it would still defend South Vietnam.
Ken Hughes, a Nixon expert at the University of Virginia's Presidential Recordings Project, said the tapes showed that Mr Nixon was "very duplicitous" with the South Vietnamese.
Mr Nixon believed the communists would eventually triumph but could not end the war before his re-election and even afterward wanted the South to sign on to the Paris Peace Accords to provide political cover, said Mr Hughes.
"If Mr Nixon had cut off US aid to South Vietnam in 1973, the entire nation would have realised that after having added four years and 20,000 American casualties to the war, he had gotten nothing," remarked Mr Hughes.
"Mr Nixon wanted the settlement because it would put a year or two between his final withdrawal and the communists' final victory. That would make it look like South Vietnam's destruction was its own fault," he said.