FANTASIES MUST HAVE fed the spirit of the American infantry-man in Vietnam just as the canned peaches he carried in his knapsack nurtured his body. Hell means no escape; but dreams the soldier can escape his fear and dread and war can become, merely, a ghastly purgatory.

That is why Paul Berlin, an intelligent, sensitive foot soldier in Tim O'Brien's novel about Vietnam, spends so much time fantasizing. On patrol, his eyes may be focused on the ground in front of him, as he looks for the enemy mine which could blow off his legs, but his thoughts are somewhere else: home, or places he might visit after the war. And on a particular night, as he keeps watch in an observation tower near the South China Sea, he conjures up a fantasy of escape to a place as distant geographically and in every other way from wartime Vietnam as it is possible to be: Paris.

The dream journey begins in reality, as a mission to find an AWOL soldier named Cacciato (in Italian, "the pursued"), a member of Paul Berlin's platoon whose own fantasy has been to walk away from the war, westward, until he reaches Paris. The search party sets off in that direction, picks up Cacciato's trail, and traps him on a small hill near the Laotian border. Paul Berlin remembers this much, a month later in the observation tower, and then his imagination takes over: What if Cacciato had slipped away, he asks himself, and we had followed him to Paris?

Well, for one thing, Paul Berlin might get a chance to meet a woman. (A soldier's fantasies are not exclusively of escape, after all.) And improbably, as suits a dream, the search team does overtake a beautiful young Vietnamese woman, a refugee, who is going their way through Laos. It's his dream, so Paul Berlin claims her. Dream journeys tend to the picaresque, and Paul Berlin imagines several episodes in this mode. Having reached Iran, the soldiers are (accurately) accused of being deserters themselves, and barely escape execution by firing squad. They hitchhike through Eastern Europe, where they are picked up by an American flowerchild. She offers them her mindless sympathy (not to mention her body), and they take her van instead, leaving her behind.

It's a long way to Paris, but Paul Berlin has a long watch to pass that night. And we pass it with him as he weaves the boredom of the watch and the horrible memories the war has produced with the potential pleasures and dangers of dreams.

Tim O'Brien was an infantryman in Vietnam, and his first book, If I Die in a Combat Zone, a nonfiction account of his year there, shows how closely Paul Berlin is patterned after himself.In the earlier is patterned after himself. IN the earlier book, O'Brien writes, "Do dreams offer lessons: Do nightmares have themes, do we awaken and analyze them and live our lives as a result? Can the foot soldier teach anything important about war, merely for having been there? I think not. He can tell war stories." War is one of fiction's great subjects, but the soldier who can tell war stories is a rarity.Merely for having been there, the soldier, like the journalist, can describe war. But the storyteller aims at something more, a knowledge so perfect that the reader can be there, too. Anyone who has read Tolstoy's account of the battle of Borodino knows that this is not too grand a claim for what fiction can do.

Cacciato approaches these heights at times, when O'Brien describes a helicopter assault or a soldier who dies of fright after being wounded by a mine or any of the deaths that Paul Berlin remembers witnessing. For example, after the partly headless body of one soldier they called Buff was carried away by helicopter, there was this to witness:

"Very carefully, keeping it steady and close to his stomach, Cacciato picked up the helmet and carried it down the ditch to a patch of high grass.

"Life after death, Paul Berlin thought. It was a stupid thought. How could it be? Eyes and nose, an expression of dumb surprise - how could this promise anything? He tried not to look. He wanted to feel grief, or at least pity, but all he could feel was curiosity. So he looked.

"He watched as Cacciato stepped over a log, stopped, and then, like a woman emptying her wash basin, heaved Buff's face into the tall, crisp grass."

This is the reality out of which a soldier's dreams of escape arise. (Several of these extraordinary realistic chapters have appeared in publications as different as Redbook and Shenandoah, the literary magazine; one was an O. Henry award winner.) But it is Berlin's dream - with its fantasy elements - which often seems out of place, hard to reconcile with the evocative realism of the rest of the narrative. Of course, a realistic account of war need not ignore the dreams any more than it can ignore the horrors. Nonetheless, the reader and writer of fiction must strike a bargain in order for the reader to be transported to Borodino, say, or Vietnam.If the writer earns his trust, the reader suspends his disbelief, as Coleridge put it. He doesn't say to himself, "This is only a story." Unfortunately, when the fantasy intrudes, that is just what the reader says. The bargain has been broken.

However, that failure is a minor one; and at its best, Going After Cacciato is a telling depiction of the footsoldier's war. Most of what we know about Vietnam we've learned from journalists, and some of the best books to come out of Vietnam are essentially about the journalist's experience there. But it is the experience of the soldier that is important. Tim O'Brien knows the soldier as well as anybody, and is able to make us know him in the unique way that the best fiction can.