WHEN A MAN returns from a war, he is placed on a pedestal and bombarded with a deadly barrage of questions. "Were you wounded?" people want to know. "Did it hurt? Did you kill anyone?" He is asked to confirm the strange notions of war Americans are bred on: that is is horrible, that it is a nightmare, but that in the end it somehow makes a better man of you.

We are thus treated to the spectacle of former-POWs throwing out season-opening baseballs and commenting on peace demonstrations and Jane Fonda movies they never saw. Survivors of distant defenses of democracy march in annual parades and grumble about respect for the flag and those that defended it. But merely for having been there, can a soldier teach any valuable lessons about war? Tim O'Brien, who has been there, says no. He can only tell war stories.

If I Die in a Combat Zone is O'Brien's personal war story. The draft caught him after graduation from Macalester College in 1968 and kept him from going on to graduate school at Harvard. Prolonged dialogue on Vietnam had convinced him that if war itself was not wrong, this one, at least, was. He and his classmates mourned Bobby Kennedy and came clean for Gene. For most of his friends -- those who had secured letters from doctors and priests -- Vietnam ended at that.

But for the Army-bound O'Brien, there were agonizing decisions to be made. He wondered if it was the morally courageous choice to abandon his country rather than join it in making unjust war.

LIKE MOST Americans who were drafted in 1968, O'Brien did not emigrate to Canada or submit to a jail term. And even when he knew he was headed for Vietnam as a grunt, he did not desert. He formulated precise plans for an escape to Sweden, but as he started to leave, he found that the futility of a lonely protest and the vagueness of the future as an exile clouded any clear moral course It would hurt his parents more, he decided, to receive a letter from him explaining his desertion than a telegram announcing his death.

So O'Brien preserved the precarious balance of order in his life, and survived his final training. He wondered if his proud endurance of those physical trials was only fear in disguise, if he was desperately ignoring his failure to say a final dramatic no to the war. O'Brien decided he was a coward and boarded a plane for Cam Ranh Bay.

The "coward" won seven medals in Vietnam, and, more importantly, survived. O'Brien trudged through swamps, and watched friends and enemies die, witnessed war crimes and simple insults to humanity. His college dialogues on the war were left far behind. " ...No one in Alpha Company knows or cares about the cause or purpose of their war," he wrote. "It is about 'dinks and slopes' and the idea is simply to kill them or avoid them." O'Brien did more avoiding than killing, eventually wangled a job in the rear, and endured Vietnam.

O'BRIEN WROTE his personal version of the war ostensibly to present a grunt's-eye-view of Vietnam. "The book isn't very pretentious," he admitted in an interview. "There's very little politics in it, for example. But I don't think the footsoldier has been properly exposed to the media. All those books on World War II -- I just don't think they accurately describe war."

But O'Brien's powerful description of his year in Vietnam does much more. Not surprisingly, it confirms many of the stories that have filtered back from hamlets like My Lai (where O'Brian frequently patrolled). Civilians are randomly murdered. Officers are routinely fragged. But more hysterically than any previous war, Vietnam evoked fear in her combatants, and the various ways the war tested O'Brien's courage is what his book is about.

Worse than bullets for O'Brien were the omnipresent mines and booby-traps that forced every step, every shifting of body weight to be a carefully considered move. No place, no hour was safe. After O'Brien had watched a few friends placed in plastic body bags, his sensitivity gave way to a determined indifference. A corpse became merely someone he had "clobbered in ping pong back in Chu Lai." Like other wars, there were too many challenges to what was perceived as a man's courage for more than grim sentimentality. "Watching friends die gnaws at you," O'Brien says. "If I had gone through World War II, I probably would have written much the same book."

But Vietnam tried men in other ways. Was courage charging a machine gun or saying no to a wrong war? Or was it merely wise endurance of an insane environment? Four years later, O'Brien is still unsure. "I was brave in one sense," he says. "I had to withstand all that crap of training. In that sense I did all right. On the other side of the coin is the moral question. If I had to do it all over, I probably wouldn't go. But back in the 60s, there were all sorts of social pressures..."

Once he had decided to endure, to go to Vietnam, there was no let up in challenges to his moral courage. O'Brien wrote of a soldier throwing a carton of milk at the head of an ancient, blind Vietnamese peasant. When asked now about his own actions, he is troubled. "I sort of wrote myself out of that scene. I just stood there and watched it happen," he says. "Looking back, I should have done something."

THE NATURE OF courage and the tenuous link between its moral and physical forms is a confusing, unexplored realm. What appear to be the most demanding alternatives -- charging or refusing to -- are actually only the simplest. If I Die in a Combat Zone poses no answers, only elucidates the basic dilemma. It is eloquent, and it is powerful, and at times, it is even bitterly funny. But it is only a war story.

O'Brien is still plagued by his inconsistency. He wonders just how brave he is, just how brave he has to be. "Do you have to be brave all the time?" he asks. "Or just 10 per cent of the time? I don't know. I'm only batting around .180 or .200. I want a chance to up my average." This, in itself, is a form of courage.