By JACK F. MATLOCK JR.

        In the endless flow of assessments, reassessments and re-reassessments of the war in Vietnam, a study occasionally appears that goes beyond a rehash of the polemics that have marked that tragic experience. Larry Berman's ''No Peace, No Honor'' belongs in that select category. Using extensive research in American archives, where some key pieces of evidence are just coming to light, as well as documentation from both Vietnams, China and the Soviet Union, Berman has built a strong case in support of the following conclusions:

    1. Richard Nixon encouraged the South Vietnamese president, Nguyen Van Thieu, to refuse to participate in the negotiations with North Vietnam arranged by the Johnson administration in October 1968, with assurances that if elected, he would provide Thieu with more solid support than his Democratic opponent, Hubert Humphrey, would.

    2. Thieu's public refusal to take part in the negotiations in Paris, announced just days before the presidential election in the United States, might have damaged the Humphrey campaign sufficiently to deliver the election to Nixon.

    3. Once in office, Nixon systematically betrayed the assurances he had given Thieu by allowing Henry Kissinger to make secret deals with the North Vietnamese regarding the political future of South Vietnam behind the back of Thieu and his government.

    4. The agreement ''on ending the war and restoring peace in Vietnam,'' signed in Geneva in January 1973, neither ended the war nor restored peace, since the fighting in South Vietnam never ceased. On almost every important point, the accord incorporated the positions held by the Vietnamese Communists for years. Even so, North Vietnam and its Vietcong allies in the South violated the few promises they made from Day 1.

    Berman is not the first to make these points, but he does so with more convincing evidence than his predecessors. He generally refrains from overstating his case. He does not, for example, argue that Humphrey would necessarily have won the 1968 election if Thieu had agreed to take part in the negotiations -- possibly he would have; possibly not. He allows that Thieu had good reasons, entirely aside from Nixon's promise, to refuse to take part in talks that granted equal status to the Vietcong. He also avoids the mistake some observers make when they assume that the negotiations contemplated in October 1968 would necessarily have brought about a peace agreement. With what we now know of the thinking in North Vietnam at the time, Hanoi would not have accepted an agreement unless Washington had been willing to make the wholesale surrender to North Vietnam's terms that Kissinger made later.

    Nevertheless, it is clear that Nixon did try to prevent Thieu from cooperating in peace talks, even as he pretended publicly to support the talks; that he gave assurances of support through intermediaries; and that he subsequently reneged on these pledges without ever admitting it.

    Berman, the author of two previous books on Vietnam, takes due note of the defense Nixon and Kissinger made of their actions. They claimed to have had every intention of continuing aid to South Vietnam after the agreement was reached and of trying to prevent Communist violations of the accord (by bombing North Vietnam). However, they argue, in the atmosphere of the Watergate scandal, Congress prevented them from providing aid or retaliating.

    This, of course, is true as far as it goes. What the excuse ignores is the manner in which duplicitous diplomacy, public lies and attempts to mislead Congress (not to speak of the Watergate break-in itself, a byproduct of Nixon's efforts to maintain secrecy regarding Vietnam) led to the collapse of Congressional confidence in Nixon's Vietnam policy.

    Kissinger's practice of diplomacy during this period can be faulted for both its substance and its manner. His detailed negotiations about the internal political structure of South Vietnam were not only a breach of faith with the South Vietnamese government, but also the result of poor judgment. His naïve belief that China and the Soviet Union could be induced to help the United States extricate itself from Vietnam stemmed from a faulty understanding of all the parties he was dealing with. The North Vietnamese resented the pressures Moscow and Beijing had brought to bear on them in 1954, when they had agreed to a division of the country, and were determined not to yield again to any attempt to limit their goal of conquering all of Vietnam. Despite their warming ties with the United States, the Chinese advised the North Vietnamese to hang tough in the negotiations. As Berman observes, ''If the China card had had any effect'' on Kissinger's negotiations, ''it apparently was to increase the pace of American concessions.''

    So far as Kissinger's negotiating style was concerned, its basic flaws were excessive secrecy, duplicity, egocentrism and hubris. It was a style that fitted Nixon's prejudices, even though Nixon at times was incensed by Kissinger's attempts to take personal credit for claimed feats of negotiation.

    Secrecy, of course, is essential in some stages of diplomatic negotiation. Unpublicized contacts between trusted emissaries can help resolve tough issues when both sides want a solution. Decision makers themselves, however, conduct secret talks under an enormous handicap: they cannot make informal suggestions without being implicitly bound by them. The situation becomes even more damaging if negotiators of high rank are found to be double-dealing. (An emissary can always be disavowed.)

    Kissinger's ''back channel'' negotiations with the Vietnamese violated all the traditional norms of diplomacy. They were secret to none of our adversaries but only to those Americans who would be required to carry out the decisions or, in the case of Congress, to support them politically. They achieved nothing that could not have been obtained at the formal negotiating table -- or, in fact, without any agreement at all. The only advantage of secrecy was to permit Kissinger to misrepresent their substance and boast of a diplomatic victory.

    But we must be fair and ask, when all is said and done, whether Nixon, once elected president, really had a choice. He inherited a war in which 560,000 American troops were directly involved and to which opposition at home was growing. A sudden abandonment of South Vietnam would have led to an immediate collapse of its government and a backlash of unpredictable severity in the United States, where the majority, usually silent during antiwar demonstrations, still favored the defense of South Vietnam.

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