THE PALACE FILE by Nguyen Tien Hung and Jerrold L. Schecter (Harper & Row: $22.50; 416 pp.)

Nguyen Tien Hung is one of those curious South Vietnamese patriots who would rather cavil than fight. After leaving Vietnam in 1958 to study and teach in the United States, he stayed away for nearly 15 years even as more and more American and Vietnamese boys fought and died to save the homeland he’d left behind. Only after the Paris Peace Accords in 1973 did he again take up residence in South Vietnam, and then as President Nguyen Van Thieu’s rather privileged economics adviser.

Hung’s failure to rally more energetically to Saigon’s standard is something worth pondering, for in this co-authored account of the war’s end, he seems determined to fault the United States for not doing enough itself to prop up South Vietnam’s losing cause. Coming from an armchair warrior like Hung, this is a dazzling piece of audacity.

His case against the United States rests on a packet of private letters--”the Palace File”--which President Thieu received from the White House during the last years of the war. In mid-April, 1975, Hung brought the letters to Washington in hopes of using them to shame the American public into supporting a three-year loan for Saigon. The ploy didn’t work. The North Vietnamese victory was only two weeks away. But Hung clearly isn’t content to let the American conscience rest easily.

The letters, signed by Presidents Nixon and Ford and published here in full for the first time, represent a mix of coercion and cajolery--promises of U.S. support coupled with threats of an aid cut-off--designed to get Thieu to back the Paris Peace Accords and other fruits of Henry Kissinger’s secret diplomacy.

Hung and his co-author Jerrold Schecter, a former Time

correspondent in Saigon, interpret the letters as a promise that the United States would come to Saigon’s aid, even with B-52s, if the North Vietnamese should violate the peace agreement. Had it not been for this commitment, they argue, Thieu wouldn’t have signed the accord. And had the United States honored its promises, Saigon might not have been lost.

There is much about this thesis that is troubling, but nothing more so than the way the authors handle this basic underlying question: Can the American people be held to secret presidential commitments that are ultimately circumscribed by law?

The War Powers Act and other Watergate-inspired legislation prevented the White House from delivering on all of Nixon’s promises to Thieu. Hung and Schecter recognize this, but conclude unsatisfactorily that the President’s word, as reflected in the letters, should have been bond anyway. This notion ignores the importance of law in our society and surely deserves more skepticism.

Even so, the authors should be credited--particularly in view of the current troubles in Washington--for underscoring the perils of sub rosa policy-making. If the Nixon Administration had publicly acknowledged its pledges to the South Vietnamese, Congress might, just might, have been more generous in subsequent aid appropriations.

Whether that would have saved Saigon, however, is scarcely the foregone conclusion that the authors seem to think it is. As they acknowledge only ambiguously, the South Vietnamese had never shown themselves capable of fending off their adversaries without massive U.S. air support. And the U.S. bombing itself has proved ineffective in crippling Hanoi’s war-making potential--a fact that quickly became apparent after the cease-fire as the North Vietnamese rebounded militarily with little additional aid from their own allies.

In view of this, it seems simplistic to suggest, as Hung and Schecter do, that the Paris agreement was needlessly concessionary, that Kissinger fumbled in allowing Hanoi to keep its forces in the south and that Thieu might have done better by holding out for better terms. In fact, nobody had won the right to dictate conditions to Hanoi, and had Thieu refused to endorse the Paris agreement, Congress would have cut off aid immediately. He was therefore not so much the betrayed ally as the beggared supplicant.

In discussing the peace agreement itself, Hung and Schecter are so hard on Kissinger that even his severest critics may take umbrage. Contrary to what they have been told by their favorite source, Nguyen Van Thieu, the agreement did not propel Saigon toward coalition with the Communists. And though it did leave North Vietnamese forces in place, it was buttressed by understandings designed to keep the two sides in balance. That these were never enforced resulted from Saigon’s perfidy, as well as Hanoi’s, though you won’t learn that from this book.

Nor do the authors fairly represent the Palace File itself. While they read the letters as guaranteeing U.S. military retaliation and even B-52 strikes in the face of North Vietnamese truce violation, the actual language Nixon used is less explicit.

The pledges of material aid are equally imprecise, and though the authors claim that Nixon never alerted Thieu that Congress’ approval might be needed for these commitments, the letters themselves prove otherwise. Not only did Nixon warn Thieu of the volatility of U.S. public opinion and “the growing difficulties in obtaining adequate aid levels from the Congress,” he also conditioned additional assistance on “the cease-fire provisions of this government.”

Equally misleading is the authors’ suggestion that aid cuts imposed after the cease-fire doomed Saigon. Undoubtedly the South Vietnamese were strapped for some materiel as Congress tightened the purse strings, but whether the shortages were as severe as the authors (and their partisan U.S. military sources) claim, remains unproven.

Nor is it clear that the South Vietnamese could have used additional aid effectively. The authors note correctly that Saigon’s military command was corrupt and fragmented, that its logistics lines were a joke, and that Thieu himself repeatedly botched crucial military decisions by refusing to issue explicit orders.

Considering all this, it requires considerable imagination to believe that any more American largess could have turned the tide.

Right after the cease-fire, Thieu might have bettered his survival prospects by cleaning house and trimming his defenses to conform to diminishing resources. But he didn’t.

For reasons of pride and personal expediency, he resisted attempts to enforce and strengthen the ceasefire and allowed his government to remain corrupt and fractious so that no one would ever challenge him politically. And despite an obvious slippage in U.S. support, he persisted in grabbing territory throughout the early months of the cease-fire, so that by the end of 1973, his hard-pressed army was dangerously--and unnecessarily--overextended.

Hung and Schecter either ignore these developments or attempt to blame the worst of them on somebody other than Thieu. The North Vietnamese become solely responsible for the cease-fire violations, and the Americans take the rap for much of Saigon’s abiding corruption.

Despite themselves, though, the authors do make clear why Thieu was finally unworthy of victory, or even an honorable defeat.

During the last weeks of the war, they write, Thieu and his adviser Hung pleaded with U.S. officials to mount B-52 strikes on North Vietnamese forces in the south. By that point, Communist troops had seized all major population centers north of Saigon, and countless South Vietnamese refugees were strung out amid the advancing enemy columns. Bombing these troop concentrations might have delayed the Communist victory, but tens of thousands of helpless South Vietnamese civilians, Thieu’s own countrymen, would have been killed. That is a measure of the leadership for which this book is an apology.