PLATOON LEADER. BQ048.4s R. McDonough Presidio. 195 pp. $15.95.

A ND A HARD RAIN FELL. By John Ketwig Macmillan. 320 pp. $18.95.

ONCE A WARRIOR KING. By David Donovan McGraw-Hill. 323 pp. $15.95.

WE ARE 17 years from the time of the largest American Army in Vietnam, 17 years from the cataclysm of Tet and from the time, also, when it began to seem that we would not win the war. A soldier who fought in that time, therefore, and who writes about his experience in the war, is now as removed from it in time as a veteran of Iwo Jima writing of his experience from the vantage point of the early 1960s.

Yet for Vietnam veterans the experience retains an elemental and terrible immediacy, something approaching, in at least one of these memoirs, an extended primal scream of shame; and in the other two a kind of detailed recollection that testifies, not to the fact that their authors' fighting was more horrible than that of other soldiers in other wars, but that their need to justify what they did to an unknowing and ungrateful nation is infinitely larger. There is, for the Vietnam veteran, no emotion tranquilly recollected. The old aphorism about enough anger obviating the need for writing talent is sometimes true. The clumsiness, the clich,es, the absence of art: these are the testament of a war both unliterary and unloved. To read all these books, indeed to read them together, is to experience something perhaps more evocative of that awful time in Vietnam than the great fictions by Americans who served in World War II can provide of that war.

Each memoir recalls a year of service in Vietnam. One, by any reckoning the best, is by a regular Army officer, James R. McDonough, who is now a lieutenant colonel. It is an account of that portion of his tour that he spent commanding a platoon in the 173rd Airborne Brigade. It is taut, pokerfaced, unador it testifies to a kind of military, not militaristic, professionalism that one might have found in the young regular officers of the Rainbow Division in World War I or in those of a Marine division in the Pacific 20 years later. McDonough does not like war. He went to West Point from Brooklyn, and from West Point, after training, to Vietnam. He is impelled by some implanted sense of duty to lead and to fight as well as he possibly can. "I could not 'manage' my platoon up a hill," McDonough writes, "I had to lead them up there. I had a mission to accomplish, and I had men to keep alive, as many as I could. I had to do more than keep them alive. I had to preserve their human dignity. I was making them kill, forcing them to commit the most uncivilized of acts, but at the same time I had to keep them civilized. That was my duty as their leader. They were good men, but they were facing death, and men facing death can forgive themselves many things."

An infantry officer must do two things: carry out his mission and assure the welfare of his men. McDonough knows that mastering his craft as a junior infantry officer is the only way even to begin to solve the problem of reconciling these two irreconcilables. His memoir is essentially an extended narrative of the military craft, of the soldier's art. And yet it is a document of rich humanity: something perhaps of the kind a young George Marshall might have written. "Here is how I set about performing my duty," he seems to be saying. His absorption in the details of his task is complete. His is the work of meticulous preparation and training, of laying ambushes and carrying them out, of selecting subordinate NCOs and calling them to account for their failures, of self-command in conditions of almost unimaginable fatigue and unvarying danger. Only rarely does he reflect on the fundamental horror: "We were a group of average young men -- young men brought up in varying degrees of a central godly ethic . . .the enemy were also young, also adherents of a humanistic faith in which killing was not the essence of life. Yet now we faced each other, obsessed with the idea of killing each other. There was no respite from it." The tone of the book throughout is unhurried, investigative, calm.

Thucydides, an Athenian, undertook to write the history of another war, knowing that it was important. McDonough's memoir is of the same school: many "levels" down, of course, but from the same perspective: here is the war, here we are, assigned to fight in it. Let me describe how we made the best of what we were ordered to do. No book by a professional American soldier has done it so well.

McDONOUGH's service in Vietnam, actually, was quite late: 1971; but his soldiers were products of the same culture that produced John Ketwig, who served in Vietnam four years earlier, the author of And A Hard Rain Fell. This is the culture of a "427 Chevelle with cheater slicks and tripower carbs, parked way at the back of the drive-in, with footprints on the headliner and beer cans under the seat . . . of flush toilets and door knobs and fishing streams." Knowing they're going to get him, Ketwig enlists. He loathes the army, and his loathing of its aims and texture produces a wild surreal account at its best as powerful as C,eline's dark writing of World War I. His perspective is precisely the opposite of McDonough's. He admires the protesters back home, particularly those he calls courageous enough to escape to Canada, which he calls a "tribute to their humanity. These people had been watching the agony of Khe Sanh on the television every night. We hadn't been exposed to much of that before we shipped out in 1967. . . Their courage was an inspiration to many of us, and throughout the war I kept a picture of a protester planting a flower in the barrel of a bayonetted rifle on the steps of the Pentagon."

Ketwig's service, mainly as a Pfc with an ordnance unit, is the familiar mixture of intense frightened boredom and Kurtzian horror: of desperate night encounters with the enemy, including one -- improbably -- in an underground tunnel beneath a whorehouse, where, stoned and disoriented, he shares a beer with four North Vietnamese. For Ketwig, sergeants are overweight bigots; drill instructors fiendishly inventive sadists; the government duplicitous, uncaring, violent and predatory; and war itself a nightmare from which only marijuana, beer, and a young woman can provide respite. He sees himself, after a year in Vietnam and another in Thailand, "devastated, ravished, defiled . . . a hollow shell."

David Donovan's (a pseudonym) war was in the Mekong Delta, in 1969; where he was assigned as an advisor to a Vietnamese village cadre on the Plain of Reeds, a lonely outpost commanded initially by a flaky American captain who, trying to frighten a Viet Cong prisoner, fires his M -- 16 and hits a cylinder of propane gas. The outpost is destroyed, the captain sent home, and Donovan put in charge. "I was now omnipotent. I knew that nothing would stop me no matter what I did. I was king. I was riding a surge of absolute power. . . I was in uncharted territory and my moral navigator was calling out urgent soundings." At one point, exhausted and enraged, he interrogates a prisoner, puts the muzzle right beside the man's ear. The bullet didn't hit him, but the sound must at least have blown out his ear drum. I thought nothing of it." But mainly, long patrolling missions in concert with naval riverine forces excepted, Donovan's was an advisory mission, and the work of organizing the village's forces, such as they were, arranging medical help for its people, trying, blindly, to keep his district "friendly."

Like McDonough and Ketwig, Donovan diagnoses and deplores the incomprehension of his superiors -- incomprehension of his problems and unwillingness to resolve them. No doubt, he reasons, it all looks good over cocktails on the roof of a club in Saigon. But in fact each successive level of command has a powerfully vested interest in making its operations appear successful; so that, by the time some distant High Command assesses the work of "pacification," all it sees is a spreading patch of blue on a large-scale map in an air-conditioned room. It sees what it is told to see; and it is told to see what toilers down the chain-of-command want it to see. As for American soldiers and their view of those whose fortunes they are supposed to be defending, a lieutenant from the Ninth Division tells Donovan, "Believe me, these (Americans) just see slopes. They don't see good or bad, they just see slopes, and they don't like any of them."

Mark Twain wrote in his memoirs that John Hay told him that, no matter what he wrote in his memoirs, or how he wrote it, his "facts and his fictions would cooperate loyally together for the protection of the reader." It is impossible to know how accurate are these Vietnam recollections in their details, how significantly time and rage or the effort to understand have altered the remembered courses of savage encounter, of the memories of bitter and exhausted and terrified comrades-in-arms. Polemical writing is at best the province of accreted detail -- dense narrative rather than commentary. And yet, if the reader is protected, armed with some sensitivity against angry recrimination and accusation, strangely, he is almost convinced by what it is trying to say. A man without the skills of a writer (the good writers, remember, were draft-exempt in that war) can nonetheless grab you and repeat, screaming, "Can't you see? Don't you understand?"; and, after a while, perhaps you do. This fall, a generation of children who had not been born when John Ketwig enlisted will enter college. If they want to know about how it was in Vietnam for Americans close to them in age, they can do no better than to read these books -- but all three of them.