IT SEEMS THAT in their final days, empires begin to turn faster and faster, careening along and over everything. As the fuses blow and the fires of decadence burn brighter and brighter they whip apart, spewing people and culture everywhere. Then the barbarians--or the Richard Nixons--ride into town. Perhaps future historians will chart the beginning of the fall of the American Imperium as that frenzied year of Tet and Chicago, 1968. Or maybe they will see it as our coming of age--in the highlands and paddies of Indochina the distinctions of war blurred into My Lai Four and the question became not just who was crazy and who was sane, but who was there and who was not. Westmoreland and LBJ were not there--they dreamed of conquering Gaul. Tim O'Brien, the ex-infantryman and former Washington Post reporter who is the author of this fine novel was, and wondering why he had not gotten on the bus to Canada. And Cacciato was marching the 8,600 statute miles that lie between the Laotian border and Paris. Paris, France.

Cacciato is the goofy kid who fishes the Lake Country, a bombed-out region where the shellholes are filled only with dirty water and corpses, or worse, pieces of them. His courage is not in question; the Company remembers how Cacciato got the Silver Star for shooting a Viet Cong in the teeth. But one day the goofy kid checks out, marching to Paris, and the squad is sent to bring him back--Paul Berlin, Doc Paret, Oscar Johnson, the Lieutenant, and the rest--but in Paris they lose him, just as they lost him all along the way. This is a wonderful idea for a novel, but even in this novel it doesn't quite happen--at the Laotian border the squad turns back, and the chase goes on only in the mind of Paul Berlin.

O'Brien talked in a recent Real Paper feature of the dreaming he did in Vietnam to keep his mind off the war. His personal daydream was to leave the war and live in a chalet in the Austrian mountains, and read. But, he said of the dilemma that caught him between fighting in a war that was morally wrong and the obligation he felt to his country and to his middle-western, middle-class parents: "My single philosophical tenet is that you can't outrun your sense of obligation--even in imagination. It's always there, you can't do it."

The novel proceeds on three complex, interlocking levels: the chase after Cacciato on the road to Paris: flashbacks that are among the best writing in the book--some of the best American writing of combat since Hemingway--and finally, Paul Berlin's thoughts one night at the observation tower where he is on guard. For Berlin, the issue comes down to courage:

He believed, like Doc Peret, that somewhere inside each man is a biological center for the exercise of courage, a piece of tissue that might be touched and sparked and made to respond, a chemical maybe or a lone chromosome that when made to fire would produce chain reactions of valor that even the biles could not drown. A filament, a fuse, that if ignited would release the full energy of what might be. There was a Silver star twinkling somewhere inside him.

Berlin is a survivor, a competent soldier who doesn't care much for soldiering, the man who escapes the daily horror by wandering after Cacciato to Paris. The epigram that starts the book--"Soldiers are dreamers," by Siegfried Sassoon--reminds us that they are, from Cacciato to Berlin, yes, even to Westmoreland, sitting in Saigon wanting to be another Grant, forgetting how Grant won battles: by throwing wave after wave of young men against the fire.

IT SEEMS FUNNY that Hemingway, who is certainly not the best American writer of this century, should have such an effect on so many young authors. Hemingway drones below the line on every page, and his themes, coarsely described as the West Point motto--Duty, Honor, Country--are also O'Brien's. But modified--if Hemingway is in some form or another the spiritual godfather of this book other influences also shine through. As Michael Herr said in his excellent book of reporting on the war, Dispatches, the crazy war in Vietnam and the equally crazy cultural revolution here both fed off the same circuit, to the point that the survivors of both of those twittering machines are almost indistinguishable from each other. "All Along the Watchtower" arcs above the prose as Hemingway does below, not Dylan's fearful version but Jimi Hendrix's scream. The prose itself is cool, boldly surreal for an American writer--magically realistic. That may be the only way an individual can capture and filter and finally understand the ultimate horror of Vietnam: stripping naked a burnt-out old man to search him for weapons, fishing the Lake Country for fish who don't live in shell craters. Even these vulgarities don't match the ones that flashed on our television screens every night for the better part of a decade--the Saigon police chief with his gun to the head of a suspect, Buddhist monks on fire in the streets of Hue, the little napalmed girl running in terror down a rural road. And so Paul Berlin, and one suspects O'Brien too, goes after Cacciato.

All along the trail there are glimpses of Cacciato, as he helps the squad out of trouble, until the final ending in Paris where the squad is left essentially where they started: without Cacciato, leaderless, without a sense of mission. The man who laughed has slipped away again, and Paul Berlin, left with his sense of obligation, climbs down from the observation tower to go back to the senseless war. Michael Herr relates in Dispatches the story of passing a blind man on a New York street with a friend who was a medic in Vietnam. Around the man's neck was a sign that read, "My Nights Are Darker Than Your Days." Herr and his friend looked at each other, almost guiltily; if you only knew, they thought. Going After Cacciato was six years in the writing; let us hope O'Brien writes again soon. He helps answer, for the men who did what they did, to the shrill facts of history.