KENNEDY IN VIETNAM. By William J. Rust and the Editors of U.S. News Books Scribners. 252 pp. $15.95.

IT'S HARD to believe that in just a few more months, a full quarter-century will have passed since that snowy January day when John F. Kennedy told the world in his inaugural address: "We shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and success of liberty."

At the time those words sounded heroic. Now they have a far more ambiguous and ironic ring. No one listening to that speech today could avoid hearing, along with its pride and dedication, a tone of overweening arrogance -- growing out of idealism, perhaps, but belligerent and dangerously ignorant of the world's frustrating complexities.

The years since the Kennedy inaugural have taught us a great deal about the limits of American power. The most painful lesson, of course, was the failure in Vietnam, a military and political catastrophe in which some of the crucial early steps were taken by Kennedy himself. Those decisions are the subject of this careful, compact and fair-minded study.

Kennedy in Vietnam uses extensive files of formerly classified cables, memoranda, intelligence reports and other documents from the White House, State Department, Central Intelligence Agency and the Pentagon. Though some of the documents were previously available, principally in the Pentagon Papers, many others have only recently been declassified and are published here for the first time. Supplementing the written records are interviews with 20 or so participants, at both the policymaking and the working levels, in the events of the time.

Most of this material was originally compiled for a multivolume Vietnam history planned, but never published, by the now-defunct book division of the magazine U.S. News & World Report. Whether the voluminous research files gathered for that project will lead to more books after Kennedy in Vietnam is uncertain, according to an official at the magazine; for the time being no others are in the works.

In this study, author William Rust (who was managing editor for the U.S. News Books project) reconstructs the Kennedy administration's mounting concern over the communists' "liberation war" in Vietnam, its growing doubts about the reliability and effectiveness of South Vietnam's President Ngo Dinh Diem and its somewhat reluctant but inexorable reliance on American military involvement, which Washington increasingly came to see as the only way to compensate for Diem's weakness.

The dilemma over Diem was the constant companion of the administration's Vietnam deliberations. Only eight days after Kennedy took office, Rust reports, Secretary of State Dean Rusk explained to a e House meeting the "extremely frustrating task" of American diplomacy in Vietnam, which was caught between supporting Diem against the communists and pressuring him to undertake political and social reforms.

The task was made even more difficult because the U.S. was unwilling to risk South Vietnam's defeat. "Because of the perceived international and domestic consequences of an American withdrawal," Rust writes, "U.S. officials could never credibly threaten the various South Vietnamese governments with the ultimate sanction -- abandoning the country to the Communists."

Those contradictions were never resolved, and Diem (like other difficult U.S. allies after him, in Vietnam and elsewhere) remained largely impervious to American proposals for reform. Eventually, Washington's frustration led to the American-abetted plot in which Diem was overthrown by his own generals and murdered, along with his brother, Ngo Dinh Nhu, just three weeks before Kennedy himself was assassinated in Dallas.

In oth the historical and the moral sense, that was the central event of Vietnam policy during the Kennedy presidency. Rust's version does not differ greatly from earlier histories of the coup, but it adds detail and documentation that make it even harder to deny a heavy American responsibility for Diem's death.

The plotters were told before the coup that the United States would not "actively" condone assassination, but, Rust writes, there was no positive message to the generals that Diem's and Nhu's lives must be spared. After the coup began, American officials trying to preserve "plausible denial" of U.S. involvement failed for 19 crucial hours to provide a plane to fly the Ngo brothers to safety, even though the plane was requested by the coup leaders themselves. Rust's conclusion is unsparing but difficult to dispute: "Although no American official desired Diem's death -- much less conspired to kill him -- the U.S. government's attempt to distnce itself from the plotting contributed to his assassination."

In ways that could not be fully grasped at the time, the overthrow of Diem fundamentally changed America's Vietnam commitment. Washington's role in the coup also bequeathed an inescapable American responsibility for what followed. The military-dominated governments that ruled South Vietnam for the rest of the war were, as Kennedy himself once said of Vietnam in a different context, "our offspring," to whose fate and ultimate defeat Kennedy's successors were inextricably bound.

In its account of all these events, Kennedy in Vietnam has the workmanlike quality of old-fashioned journalism. It is economically written, unpolemical, unemotional almost to the point of being antiseptic and sticks closely to the available facts, almost never venturing into conjecture or speculation. One puts this book down with contradictory feelings: on the one hand, that this is as sound and reliable an account as we are likely to have of U.S. deliberations and actions regarding Vietnam during the Kennedy years; on the other, that the story is still incomplete.

THERE ARE two reasons, I think, for that lingering sense of incompleteness. One is that because it relies almost entirely on U.S. government documents, Kennedy in Vietnam unavoidably reflects America's blindness to the place itself. In the official records of American actions, and thus in this study as well, the Vietnamese and their traditions and political and historical realities are only the faintest of shadows. "Who are these people?" Kennedy once asked his advisers. He meant the militant Buddhists, but might as well have been speaking about the Vietnamese in general. American political and military leaders, unable to see anything beyond their own power and technology, never succeeded in answering that question; neither does this book.

The second mystery has to do with Kennedy himself. No matter how complete a record we have of what his advisers said and wrote, what decisions were made, and how they were carried out, we still can never know what Kennedy's own thoughts and intentions really were.

For that reason, any study of his Vietnam policies, even one as careful and well-documented as this is, has a blank spot, an unsolved riddle, at its very heart. All we can know is the bleak result of his efforts, which "bequeathed to his successor," as Rust observes in Kennedy in Vietnam, "a larger and more hazardous commitment to an alien land of no inherent value to the United States."

What grew out of that commitment was a traumatic failure that hardly any American of Kennedy's self-confident era could have foreseen, and which makes his brave inaugural promise sound, in today's ears, like a hollow mockery.