PERHAPS it has something to do with adding a prefix to your creed, but Norman Podhoretz, high priest of Neo-Conservatism and distinguished editor of Commentary magazine, is a far better anthologist of other people's thinking than he is an articulator of his own. The title of his latest book, Why We Were in Vietnam, offers the promise, at least, of a serious search for the answer to one of the most tormenting questions of our times, some serious digging for the roots of involvement, the prevailing casts of mind, the public mood.

But just as Podhoretz has you up on your tippy toes, straining for a careful analysis of the policy-making process, the roads taken (and not taken)--the warning signs that might help us recognize a wrong turn before once again becoming hopelessly, horribly lost--he wanders off into a tendentious argument over why we lost the war.

It is familiar, inflammatory stuff, with predictable villains: Susan Sontag and Mary McCarthy, among the dissenting spectators; John F. Kennedy and William Fulbright among the participants. The media did it. The academicians did it. And so on.

Some of it makes interesting reading. A lot of it is lively. It will comfort the converted, irritate the unreconstructed. But it is bad history, and poor scholarship. It is, in short, one more rummage around the wreckage, selectively and self-servingly examining bits and pieces and reassembling them in an effort to prove a particular theory of the case.

To get an idea of how close Podhoretz gets to answering the question of why we were in Vietnam, I offer in evidence his closing paragraph. He begins it by juxtaposing a fragment of a 1977 pronoucement by Jimmy Carter on the "intellectual and moral poverty" of our Vietnam effort with Ronald Reagan's praise of it, in the 1980 campaign, as a "noble cause." He tells us that Reagan was accused of making a "gaffe." (That Reagan was also widely praised for his courage and cheered by his audience at a convention of Veterans of Foreign Wars is not noted.) Having thus defined the choices, Podhoretz takes his pick, after a fashion, in the final mind-bending sentence of the book:

"Fully, painfully aware as I am that the American effort to save Vietnam from Communism was indeed beyond our intellectual and moral capabilities, I believe the story shows that Reagan's 'gaffe' was closer to the truth of why we were in Vietnam and what we did there, at least until the very end, than Carter's denigration of an act of imprudent idealism whose moral soundness has been so overwhelmingly vindicated by the hideous consequences of our defeat."

Well, now, what have we here? Reagan was more right than Carter--that much is clear, though just how much righter is not, so we don't exactly know where Podhoretz comes out. No matter; it appears that the idealism that drove us into a losing venture was imprudent, which is to say, according to Webster's, "without thought of the consequences, lacking in judgment, rash." We were in over our intellectual and moral heads, so to speak. But the hideous "consequences," which we were rash enough to think about, made our lack of judgment "morally sound."

The injuries suffered by innocent bystanders as a consequence of an accident caused by reckless driving, in other words, vindicate the fundamental decency of the driver. Or have I got that wrong?

Never mind. Early on, Podhoretz declares that, "in short, it seems reasonable to conclude that the only way the United States could have avoided defeat in Vietnam was by staying out of the war altogether." So much for the "noble cause"; Podhoretz himself concedes that Indochina could not have escaped its ultimate grim fate without us.

But John F. Kennedy, being "the kind of man he was and not the kind of man Eisenhower was," would not have had the guts, to put it bluntly, to face up to any such limits on American power, Podhoretz argues. And so "the United States, under Kennedy, for all practical purposes and in all but name, went to war in Vietnam."

Leaving aside a good deal of revisionist poking around in the origins of the "containment" doctrine (which he purposefully misread to the discredit of George Kennan in an earlier book, The Present Danger, and does so again in this one), that's pretty much the Podhoretz theory of why we were in Vietnam. Podhoretz patently has no use for Kennedy; he digs deeply into Kennedy's Senate speeches in the 1950s to explain his performance as president in the 1960s. Even when Kennedy does something right, he's wrong. As illustration of how animus skews Podhoretz's argument, consider this passage: "Kennedy was . . . acting prudently when he decided to go in 'slow and small' but like the war itself, it was the wrong prudence at the wrong time and in the wrong place." Wrong prudence?

Because he so badly wants to believe that Kennedy started the war, he is more than ready to play fast and loose with "facts" to make his point. Item: His offhand assertion that "unlike Eisenhower, (Kennedy) did in fact wind up sending 16,000 American troops" to Vietnam. The implication, of course, is that Eisenhower sent none--that Kennedy was breaking new ground, and thereby leading us into war.

That is simply untrue. By the time Kennedy took office, the United States was up to its knees, at least, in the Laotian quagmire, airlifting military supplies to our man, General Phoumi; SEATO's terms and Eisenhower administration pledges had already committed the United States to help resist any aggression threatening South Vietnam.

Podhoretz speaks of Eisenhower's decision to "stay out" of Vietnam, a reference to his refusal to send combat troops in 1954. But that's not the test of when intervention is, so to say, conceived, and still less is it an honest measure of Eisenhower's role. In 1954 he wrote a famous letter to South Vietnam's President Ngo Dinh Diem, promising "American aid" and laying the foundations for Ronald Reagan's "noble cause." Eisenhower told Diem, "The purpose of this offer is to assist the Government of Vietnam in developing and maintaining a strong viable state, capable of resisting attempted subversion or aggression through military means."

Ten years later, Lyndon Johnson was to read Eisenhower's letter at a press conference and pronounce it as a "good letter then and . . . a good letter now . . . our commitment today is just the same as the commitment made by President Eisenhower to President Diem in 1954-- a commitment to help these people defend themselves." Johnson didn't think Eisenhower had set an example of doing "nothing" and neither could anybody else who bothered to read the record.

Not only had the principle of providing military advisers been established, so had the practice. Harry Truman had actually sent 128 American military advisers to support the French in Indochina in 1950. Eisenhower increased the figure to 692 by January 1961. (Podhoretz only tells how many were there by the end of 1961.) Kennedy raised the number to 16,263. But it was not until March 1965 that the first American forces organized as combat units were dispatched--by Lyndon B. Johnson. So under what president would you say the United States "went to war in Vietnam?"

That's really the whole point--the furtive, incremental, sometimes unacknowledged, often deceptive nature of our gradual entanglement--and Podhoretz misses it entirely; one almost has to believe he wants to miss it. He is, for example, flat wrong about another critical turning point; this time the animus is aimed at another favorite liberal whipping boy, former senator William Fulbright, who floor-managed the Gulf of Tonkin resolution.

Podhoretz contends that Fulbright, in interpreting it to troubled colleagues, "pointed out that the resolution would indeed empower thae President to involve the United States in a major land war in Asia." In fact, he did just the opposite, as a reading of the Congressional Record clearly shows. Under heavy pressure from senators John Sherman Cooper, Daniel Brewster and others, and confronted with an amendment from Gaylord Nelson seeking to specify that no such authority was being granted the president, Fulbright repeatedly sought to ease any anxieties on the question of using ground forces. He said, "There is nothing in the resolution, as I read it, that contemplates it." And again: "I do not interpret the joint resolution in that way at all." He said he would accept the Nelson amendment if he thought it was needed and his reassurances were sufficiently reassuring to Nelson; the amendment was withdrawn.

When Podhoretz is as wrong or as misleading as he appears to be so often on matters of fact that bear directly on the central argument of his book, it seems to me he is practicing revisionism--with a vengeance. And this strikes me as all the less excusable when, by leaving his own record of opposition to the war out of the book while denouncing any number of those who shared his reservations at the time, he is even practicing revisionism on himself.