Earlier this year I was invited by the BBC to visit the last surviving British soldier who fought in the trenches during the first world war. He's called Harry Patch and lives in a nursing home in Wells, Somerset; in 1917, aged 19, he was a Lewis gunner in the Duke of Cornwall's Light Infantry and took part in the battle of Passchendaele. I weighed the invitation in my mind for a split-second, then said yes.

I'd grown up with the army at the borders of my life - my grandfather had fought in Flanders in 1918, and my father in France and Germany from D-day until the end of the second world war. Although neither of them had shown any interest in poetry, my own devotion to it had first been stimulated by reading Wilfred Owen and Siegfried Sassoon at school. I'd written my graduate thesis and then a book on Edward Thomas, who was killed at Arras in 1917. I'd visited the battlefields of that region several times - once with my father, on the 60th anniversary of the D-day landings. I'd edited an anthology of first-world-war poetry. Meeting Harry, I thought, would connect the writing to its circumstances in a uniquely powerful way.

And so it did. After a train-ride to Bristol, then a drive along the misty switch-back to Wells, I found Harry's nursing home among the suburbs. The staff told me how to get the best from him: he might be amazingly robust for someone 110 years old, but 110 was 110. I should speak up, talk slowly, repeat myself if necessary. The more advice I was given, the more sharply I sensed the difficulty ahead. For the past several years Harry had been seeing radio people, TV people, historians and well-wishers at a pretty steady rate. Although he told everyone he was "just an ordinary chap", and "not a hero", he'd come to be seen as just that. It evidently made him feel awkward, as well as pleased. Equally obviously, he'd developed a way of protecting himself by sticking to the same few stories - stories I'd already come across in the autobiography he published with the help of his friend Richard van Emden, The Last Fighting Tommy. My most difficult job would be to surprise Harry back into his old self; the danger would be that this might distress him.

Lights, cameras, action. Harry was brought into the room in a wheelchair - little and frail but, given his great age, astonishingly spry-looking. Blue blazer and grey trousers. A sparrow-body. A twinkle in his eyes. Medals pinned to his chest. I shook his hand, then held it for a moment. I had expected to be moved, but not this much. The fingers felt fragile as twigs; 91 years ago they had picked up a machine gun and aimed it across No Man's Land. For a moment I couldn't speak. Harry was grinning at me. He'd seen this kind of thing before.

We started talking and, sure enough, Harry's conversation fell into the same shape as the autobiography: his childhood in Combe Down outside Bath, the initial reluctance to join up in 1914, conscription, Passchendaele, the shell that killed three of his mates in the Lewis team, his own wounding and recovery in Blighty, his work as a plumber, then the long later part of his life, in which he was nearly killed again, serving as a fireman in the Bristol blitz. Harry's voice was very low, almost worn out, and I had to bend close to catch what he said. There were lengthy pauses, too, before he answered some of my questions. To start with, I thought these were to allow him to collect his memories. Gradually I realised they were to let him collect himself. Even though so much time had passed, some of his memories still appalled him.

Or maybe they had grown even more appalling over time. Harry fought in France for just a few months before he was wounded and sent home but, partly no doubt as a result of being asked to remember the war again and again, and partly as a result of some natural process, it was clear that his war experience lay at the top of his mind like a still-lethal grenade. One scene was especially difficult for him to describe - when he went over the top and stumbled on the body of another soldier, a boy, really. "He was ripped open from his shoulder to his waist by shrapnel, and was lying in a pool of blood. When we got to him, he looked at us and said, 'Shoot me.'" He was beyond all human help, and before we could draw a revolver he was dead. And the final word he uttered was 'Mother.'"

By the time our conversation ended, I had forgotten the cameras and all the other paraphernalia of filming. Harry's whispered descriptions, still amazingly vivid with detail and compassion, had drawn me completely into his world. Various misconceptions had been corrected: how, for instance, he and thousands of others had been reluctant to enlist as soon as the war broke out - I'd been raised on the idea that everyone rushed to slaughter, like they do in Philip Larkin's poem MCMXIV, innocently thinking the war was "an August bank holiday lark". Or how much human comradeship survived long into the war, regardless of nationality: Harry was always careful to shoot his enemy in the legs "and no higher" unless he thought his life was in danger.

These felt like valuable corrections to received wisdom about the war. Even more fascinating was hearing how everything in Harry's life before 1914 now seemed coloured by the-war-in-anticipation (memories of creeping along vegetable-trenches as a child), and how everything after the war had been affected by it in recollection (having to ask nurses not to turn on the light in the airing-cupboard opposite his room, because the flash took him back to the trenches). One way or another, the few months Harry served in France now entirely dominate the 1,300 months he has lived elsewhere.

Before we said goodbye, Harry breathed me a joke: he thought that because he'd been alive for such a long time, he might as well go on for ever. What he meant, in all modest seriousness, was that he knew his own value as one of the very few who are still able to say, "The war was like this; I was there." This, in turn, means that he is also one of the very few whose eyes we can look into and imagine we see what they saw, whose hands we can hold and feel they are leading us back through time. To sit in his company is to feel the flow between "then" and "now" is unbroken.

And when Harry and the others are no longer with us? Then our sense of the war as "history" will become different, simply because it will lack this particular connection. But it will be a subtle difference, because it's already abundantly clear there's no danger of the war being suddenly forgotten, or made to seem irrelevant to our sense of what Europe and the world has to avoid repeating. In fact, during the last generation or so, and for reasons that have to do with much more than the survival of Harry and a few other veterans, the first world war has been identified by common consent as one of the great turning points in our history.

In the immediate aftermath, people did what they could to put it behind them - as they did again after the second world war. (My own father, in a way that was typical of his generation, said almost nothing to me about his life in the army, no matter how much I prodded him.) But in the long western European peace since 1945, the first world war has loomed larger and larger in our imaginations. It was once described as the war to end all wars. Over time, it has become the war by which most others are measured - in spite of all the obvious differences in weaponry, motive, duration, everything. In the process, it has also become more and more clearly the event which made us "modern" - both in the sense that it accelerated the growth of our democratic structures, and loosened old class strictures, and because it made the whole population familiar with barbarity, suffering and loss on a scale never seen before. Ninety years on from the Armistice, we look at the events of 1914-18 and think we are examining our national psychic wound.

That's why Remembrance Day parades and ceremonies are given so much press and other kinds of attention these days (perhaps even more attention than previously, but these things are hard to measure). They exist to commemorate the dead of all wars, but they invariably revolve around images associated with the trenches - the heart-jolting pictures of people like Harry Patch floundering in the mud, or scrabbling over the lip of a trench and almost immediately being shot down. And throughout the rest of the year they are fed by other elements of national life. By the pathos and ubiquity of the large-scale memorials in our cities, and the smaller monuments in our villages - often recording the deaths of several members of the same family. By the way poetry of the first world war is drip-fed from the national curriculum into almost all our children as they become teenagers. (To the extent that even the best poets of other wars, such as Keith Douglas, are not studied at all, or made to seem somehow less good, because they don't conform to the criteria of war poetry established by Owen, Sassoon, et al.)

All these are reasons for thinking that when Harry Patch is no longer with us, the Great War will keep its eagle-grasp on our imaginations. Unlike the Hundred Years War, or the Napoleonic wars, it's feeling of closeness is continually refreshed by the monuments that stand at the heart of our communities, and by the fact that very many families cherish the memories of ancestors who were involved. Unlike more recent and contemporary wars, shocking as these are, it still feels on our doorstep, recollected in landscapes we recognise, and involving our neighbours as well as ourselves.

A good many poets writing after the first world war - sometimes long after it - have tried to catch this combination of immediacy and distance. Ted Hughes and Michael Longley are both especially good at it. Longley, indeed, is evidently so haunted by memories of his father's experience in the trenches, and by his own sympathetic identification with Edward Thomas, that he often writes about the war as if he had fought in it himself - and feels continually a prey to its images and remembrances. This is his short poem Pine Marten, for instance:

That stuffed pine marten in the hotel corridor

Ended up on all fours in nineteen-thirteen

And now is making it across No Man's Land

where

A patrol of gamekeepers keeps missing him.

In his curiosity about the war, and his skill in showing how it touched and touches all aspects of life, and not just the lives of soldiers in the frontline, Longley is exemplary. His poems tell us that we are all survivors of the war - not just because we might happen to have relatives who fought in it, but because its footprint is still visible somewhere near where we live, and because our sense of good fortune at living in a peaceful Europe is constantly animated by evidence from elsewhere that things could be otherwise. He confirms our communal sense that the first world war looms at the threshold of how we think about ourselves as citizens in our own later world. It is part horror-show, part cautionary tale, and partly heroic example. That's why Harry Patch and the few other survivors are so important to us. Even in their frailty, they make these things intimate. We won't entirely lose this when they are gone, but we will have to work harder to find it elsewhere. In the stones, and statues, and archives, and exhibitions, and, on Remembrance Day, in the notes of bugles calling from sad shires.

• On Saturday, the Guardian begins a unique week-long series on the history of the first world war, in the form of seven free 32-page booklets, with contemporary essays on the key themes of the war plus extracts and poems from writers including Ernest Hemingway, DH Lawrence, HG Wells, Siegfried Sassoon and Wilfred Owen.

· This article was amended on Saturday November 8 2008. Harry Patch, the last surviving British soldier who fought in the trenches during the first world war, is 110, not 109 as we originally said several times in the article above. This has been corrected.