It all happened more than 30 years ago but US writer Tim O'Brien, author of the classic war memoir, If I Die In a Combat Zone, has not forgotten his year in Vietnam. "It has stayed with me. It is part of me," he says matter-of-factly. "No, I'm not exactly spooked by it; but I still have my nightmares. No one who was there can ever forget. It haunts you." He stares into space briefly and continues: "I think that's true of anyone who has been to war."

In common with fellow veterans, who include a formidable generation of outstanding American writers such as Robert Stone, Tobias Wolff, Larry Heinneman, and Michael Herr, O'Brien agrees his experiences in the army helped make him a writer. O'Brien, whose work includes the 1979 National Book award winner Going After Cacciato and one of the finest war stories of all, The Things They Carried (from the novel of the same name), says: "I always liked the idea of writing, from the time I was a kid. But liking and actually getting down to it are different."

Vietnam stuck in the collective throat, and it took about a decade for some survivors to write about it. "That's true for many of them. But I got started pretty quickly. I wanted people to know," he says with a flash of zeal. Unlike many writers, O'Brien never quotes himself and rarely refers to his books, agreeing that many of the questions put to him may be answered as easily by words he has already written as by any answer he can provide. "Though," he qualifies this slightly, stressing, "every book I've written is a story."

Watching O'Brien as he speaks about the things war does to a man's concept of himself and others, brings to mind rhetorical questions asked by him in If I Die in a Combat Zone: "Do dreams offer lessons? Do nightmares have themes, do we awaken and analyse them? . . . Can the foot soldier teach anything important about war, merely for having been there? I think not. He can tell war stories." In conversation, he is far less eloquent, less confident than he is in print. His prose is elegant, fluid and rhythmic, often formal. Nor could anyone accuse O'Brien the man of being a laugh a minute, but the humour which so brilliantly underlines the offbeat satire of Going After Cacciato is absurdist, almost surreal.

Ostensibly the story of the Third Squad's attempt to recapture one of its men, Cacciato, who having decided he would rather be in Paris, has set out to walk some 8,600 miles to get there, it is episodic and illusionary. The cryptic narrative which develops is a magic realist odyssey of fact and imagination replete with dazzling set pieces: "Billy Boy Watkins was dead, and so was Frenchie Tucker. Billy Boy had died of fright, scared to death on the field of battle . . ." At times as hilarious as Heller's Catch-22, it is also very different - more subtle, more unexpected.

Small, compact, O'Brien wears his habitual baseball cap - this time bearing a team name no one seems to have heard of - and a broad, apologetic grin intended to counter the fact that he is very serious. Sitting in a smart Dublin hotel lounge, a piano accompanying the clatter of afternoon tea, he is obviously not a native. He has a strong Minnesota accent and sounds a lot more homespun than he is. Likeable if initially edgy, largely due, as he later explains, to the helplessness caused by temporary post-flight deafness, he displays none of the defiance or brittle intensity shared by many of his generation. His cynicism is more gracious, at times almost questioning. There is no denying he exudes a thoughtful anger mingled with mild disbelief and few illusions.

At 53, he looks younger than his years, the Ransom Everglades cap no doubt adding to the youthful image, as does his self-effacing demeanour. Who or what are the Ransom Everglades? "It's a school I visited. They asked me to wear the cap." O'Brien remains the shy college kid; polite, sensitive, apparently untouched by writerly arrogance, and readily praises John Updike, Tobias Wolff and Michael Herr as "geniuses". Genius is a word he uses freely. "We have some great novelists in America. It is a good place to be a writer in." He has no children; he is not married. It is as if he has dedicated himself to the business of telling stories, and is more driven by a need for answers than by ego.

"Some people like my books, others hate them all. It's always been like that. I'm a quiet person, all the publicity stuff has never come easy to me." His new novel, Tomcat in Love, a leisurely saga about a philandering abandoned husband obsessed with his ex-wife while remaining open to any women crossing his path, is not typical of his work and he agrees it is a deliberate bid to "get away from my serious side".

The opening passages exploring young Tom's reluctant discovery of his father's shortcomings - when a mud turtle eventually arrives home instead of a long-promised aeroplane engine - are the most memorable. At the same time, the narrator begins to realise his pal Herbie is a dangerous, seething mass of rage. While the adult Thomas Chippering is less likeable, O'Brien ensures that he expresses himself in language fitting a professor of linguistics. While he frequently stresses that as a writer he is primarily concerned "with issues of the human heart", his fiction is neither reactive nor emotive, it is essentially meditative, deliberate, poetic and quietly urgent, marked by his precise attention to detail.

Sometimes his meticulous intensity can fail him, as it did throughout the crudely polemical and densely psychological In the Lake of the Woods (1994). In it O'Brien explores the strange relationship between an ambitious politician, crushed by electoral defeat, and his long-suffering wife. The only passages which convince are those chronicling the evils perpetrated by soldiers in Vietnam.

He never wanted to go to the war and had spent a lot of time at college denouncing it. Shortly after graduation, he was drafted and admits he considered taking "a quick trip to Canada" and also toyed during his military training with desertion. What stopped him? "A lot of things. Not love of country, or patriotism. I disagreed with the whole mess - we should never have been there in the first place. No, what stopped me running away was thinking of what it would be like for my father, if he overheard someone in the neighbourhood, in a street or somewhere, say: `hey, did you hear about the O'Brien kid? Dodged the draft and ran off to Canada' - I knew it would hurt my father, not that he believed in it either and I knew he was frightened for me, but I couldn't have him feel ashamed because of something I had done."

His father served in the South Pacific and saw action at Okinawa and Iwojima. "He never talked about it and we didn't ask him. But it was only in later years I read some articles he had written about the war. They were very good, beautifully written and sad." O'Brien, born in 1946, is fourth generation Irish and admits to knowing nothing about his family connection with this country beyond that. "This is my first time here. I'm ashamed to say I don't know what part my ancestors actually came from. I keep meaning to find out." He is the eldest of three, "I have a brother and a sister. My dad worked in insurance, Mom was a schoolteacher, and I grew up in a small town."

Worthington, Minnesota, is part of the Midwest's vast prairie land, a place he describes as "very flat, beautiful, empty" and, with a smile, "sort of boring, I wanted to leave". Life in Worthington offered few diversions aside from the annual pre-Thanksgiving turkey walk. "Once a year all the turkey farmers would come into town, driving their trucks with their turkeys on board and then they set them loose and all those birds would walk down the main street." The image he evokes of turkeys on the march is amusing, if it does fall somewhat short of a true spectacle. "Well, it's not that spectacular," he mutters gravely. He studied philosophy and political science at Mulcaster College in St Paul. He says he wasn't much good at sport, "I always enjoyed it, though." Were books important to him? "I loved reading and as a kid I read lots of stuff, the usual junk. But I think at that age it isn't really so important what you read, it's that you're reading."

While he stresses he writes about the problems of the human heart rather than war per se, he has not been untouched by great war novels. For him, the finest remains Crane's 19th-century classic, The Red Badge of Courage. Hemingway's A Farewell to Arms provided him with an early hero, "and I love, still love, Catch-22. I think it's a work of genius, as is Despatches."

However much O'Brien criticised the war, being drafted was inevitable. He describes the day he returned home and immediately sensed "something was wrong". His parents were having lunch. "They were very quiet. My father showed me my draft papers and said nothing." Naturally, his mother was upset but his father's fear remains uppermost in his memory. Military training at Fort Lewis alerted him to the active dislike some career soldiers traditionally direct towards college boys like him. "One of the things you soon notice in the army is the fact there are some men who love the army; others who just see war as a way of indulging their aggressions. Some of the guys really believed in it, they couldn't wait to get out there and kill - and when they did, they enjoyed killing."

Terror remains his abiding sensation of being in Vietnam. "Every minute you were conscious that in the next you could die. Each time you survived there was this incredible feeling of relaxing - only to tense up right away, waiting to start panicking all over again about being killed." He describes the monotony of marching from one village to the next and then back again. "All the time seeing the hostility of the people, feeling their hatred of us." His war took place in the Quang Nai province where he arrived exactly a year after the My Lai massacre.

Vietnam was "so beautiful, so deadly". Death could come at any minute. He describes the day he stared at it. "Suddenly I looked, a hand grenade had landed right beside me. I froze and watched and waited." He heard the explosion before he felt it. Aside from being sprayed by shrapnel, he was not seriously wounded. "I was saved by the radio I was carrying."

There was also the fact of killing in order to survive. "You never forget firing a weapon into a space that is filled by someone who can kill you. But I suppose I was more concerned with surviving than killing. You just want to live and yet you see your friends die beside you; see legs blown off, faces exploding, bodies collapsing." After the frenzy of crossfire he was always struck by the silence "and the fact I was still alive".

Looking back on his time there, in so many ways a blur of chaotic memories and complete blanks, he vividly remembers specific moments; episodes such as the moon landing. "There I was in a field in Vietnam. It was July 1969 and I heard someone shout, `we're on the moon', and a whole lot of cheers followed. I remember thinking to myself how odd it all was, `we've got these guys on the moon, why can't they get me out of here?"'

How did he feel on leaving Vietnam? "I was happy to be alive," he says simply. Looking back on that period, he refers to the various war movies and the way Hollywood has at times chosen to glamorise the war which so many Americans remain ashamed of. "When I saw Saving Private Ryan, I just felt, `Here goes another attempt to justify the war'. All that movie really is, of course, is a vehicle for Tom Hanks". . . O'Brien stops abruptly and stresses, "but whether I'm talking about a movie or a novel, I should say, I'm not really that concerned with the message, it is the art which interests me." He then adds: "I thought Full Metal Jacket was good. That showed pretty much what it was like out there. So did Apocalypse Now."

During the flight back to the States, O'Brien changed his clothes and reverted to being a civilian, all except for his feet. As he observes in If I Die in a Combat Zone, "Much as you hate it, you don't have civilian shoes, but no one will notice." More than his footwear went unnoticed. Soon after his return he enrolled in a graduate course at Harvard to study government. "No one there knew I had been in Vietnam. I didn't care. I didn't want to talk about it." Few did. Vietnam had no heroes, at least as far as the public was concerned. "You either got back or you didn't." There is a great line in Going After Cacciato when a character asked "who was Eisenhower?" replies, "Nobody. A hero."

At Harvard O'Brien soon realised he was determined to be a writer. He left early and worked as a national affairs reporter on the Washington Post. If I Die in a Combat Zone was well received on publication in 1973. By then Nixon's impeachment was history, the fall of Saigon a reality, and O'Brien was at work on his first novel, Northern Lights, which was published in 1975. It is a strong, suspenseful, conventional narrative about two brothers, one an embittered Vietnam veteran, both at war with themselves and each other, set in the hazardous winter wilderness of Minnesota's Arrowhead country. It impressed critics and confirmed he could write strong dialogue as well as record reported speech as he had in his war journal. Even so, O'Brien was not prepared for the success of Going After Cacciato in 1979. Twenty years on, he recalls his surprise on winning the National Book Award ahead of John Cheever's Falconer and John Irving's The World According to Garp. For all its originality, one of the aspects of Cacciato which most impresses is its confidence.

O'Brien's fiction tends to move in self-contained sequences. Nowhere is this better demonstrated than throughout The Things They Carried, which was short-listed for the 1990 Pulitzer Prize, while the much anthologised, elegiac prose canto from that novel appears in the three major collections of American short fiction published in the last decade: The Granta Book of the American Short Story, edited by Richard Ford (1992), The Oxford Book of American Short Stories edited by Joyce Carol Oates, and The Picador Book of Contemporary American Stories edited by Tobias Wolff (1993).

"They carried diseases, among them malaria and dysentery. They carried lice and ringworm and leeches and paddy algae and various rots and moulds. They carried the land itself - Vietnam, the place, the soil - a powdery orange-red dust that covered their boots and fatigues and faces. They carried the sky. The whole atmosphere . . . they moved like mules . . . but it was not battle, it was just the endless march, village to village, without purpose, nothing won or lost."

For O'Brien, a good war story is not ultimately about war, it is about the burdens of the human heart. Most of his characters to date have been touched by their experiences in Vietnam; it is his major theme and one likely to endure in his work. He is not a natural performer, yet when he settles down to it, he gives the impression of being able to talk forever. His mind is full of images, memories and observation. Few writers create a stronger impression of possessing such an intense need to write everything down. It is as if the intensity of his experiences will evaporate if he doesn't.

Outside the hotel, a man is standing and casually remarks, "It's gotten colder." His accent says he is from the Southern States, "Yeah, Atlanta. Who was that guy in the cap? Should I know him? He's a writer isn't he?" On hearing that O'Brien's subject is Vietnam, the man, intrigued by my reference to O'Brien's candid attitude towards draft dodging and plans to desert, remarks, "I was there. I wanted to go. Went at 17. I served two tours. My brother wanted to go as well but they would only allow one per family in case we both got killed."

It is an odd encounter, yet another of America's forgotten heroes remembering Asia on a Dublin street corner. Although a year younger than O'Brien, he looks 20 years older. Is he bitter about having been in Vietnam? "Not about going. I was proud to serve my country. But I sure as hell am about what happened when we got back. Nobody cared. We were invisible. There was lots of celebration for the guys after the Gulf War, but nothing for us. It's a long time ago, but no one who was there forgets. We're all like survivors from some secret society."

That phrase, "survivors from some secret society", encapsulates O'Brien's parting comment about Vietnam. "It was not just the war. It was a time of protest, politics, feminism. All that 1960s thing: the music, the drugs, the sex. It was a time of chaos and madness and Vietnam . . ." He shrugs as if to say, that too was just another part of a sick, strange jigsaw no one has ever quite figured out.

Tomcat in Love by Tim O'Brien is pulished by Flamingo at £16.99 in UK.