The First World War does not grow old, as other wars grow old. Age does not weary our memories, even if the years condemn. Here is a great conundrum. Ninety years ago this month, the guns fell silent in the war that failed to end all wars. The last poilu – the last of 8,410,000 Frenchmen to be mobilised – died in March. Astoundingly, there are six British and British Empire veterans, still living. Their ages range from 107 to 112 – the last patrol of an immense host of 8,904,467 soldiers, sailors and airmen.

The First World War is passing over the horizon of living memory. With next week's 90th anniversary of the armistice of 11 November 1918, it is right, surely, to bury the Great War. Naturally, we should continue to study it as history. However, hasn't the time come to re-file the years 1914-18 in the same card-index of collective memory as the Crimean or Napoleonic wars: fascinating, terrible, but no longer linked umbilically to our own guts, to our everyday lives?

The answer, it seems, is no. And it comes not from historians or politicians or journalists, but from ordinary people, and especially ordinary British people.

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Martin Middlebrook, the first British historian to chronicle the war from the viewpoint of the common soldier, says: "After the 80th anniversary [of the Somme] in 1996, I would have told you that two things were inevitable. We will see declining numbers of people at future commemorations. Interest in the war will gradually reduce. The opposite has been true."

There are now more British visitors to the Commonwealth War Graves cemeteries in France and Belgium than ever before. The nightly ceremony of the playing of the Last Post at the Menin gate in Ypres might have been attended by a handful of people 30 years ago. Now, there is a sizeable crowd each night.

Memories of the war refuse to die but the physical traces of the great conflict are, like the survivors, fading away. As a young man, in July 1979, I cycled through the battlefields of the Somme. I was surprised by the rolling beauty of the countryside. I had imagined that the 1914-18 war had occurred on a vast, featureless plain. A couple of years later, living in Brussels, I began to visit the Flanders battlefields around Ypres. I discovered the featureless landscape of my imagination.

In both cases, I was astonished to find how many clear traces of the battles remained. Remnants of the old trench lines – then 60 years old – could still be seen in places, snaking through the fields of shining wheat and around the smaller fields of shining, white graves.

In the last 30 years, most of these remnants have been erased by the creation of larger fields and farms and the use of heavier agricultural machinery. On the edges of battlefield towns such as Albert, Arras and Ypres, industrial estates, suburban sprawl and motorways have been scribbled over parts of what was still an almost virgin landscape in 1979.

You can still find scraps of pock-marked, overgrown battlefield in woodlands or in the memorial parks, such as Beaumont-Hamel and Vimy Ridge. The shining white cemeteries are, of course, still there, beautifully preserved by the Commonwealth War Graves Commission and by their French and German counterparts.

The poignancy of this "disappearing" landscape – especially the part of it around Ypres in south-western Belgium – has been captured masterfully in these images by the German photographer Stefan Boness.

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In one image, a golf hole stands beside a small British cemetery that contains the graves of men who were killed a few yards away. In another, a factory chimney peeps over the parapet of a British trench, reconstructed in absurdly neat concrete. Perhaps most poignantly of all, a freshly ploughed field recalls the mud in which the British and German armies floundered from June to November 1917, as they fought over an insignificant village called Passchendaele. I was at just such a field with Martin Middlebrook two years ago. In the lowest corner – to which water naturally flowed – the historian bent down to pick up a tiny metal ball: shrapnel from a fragmentation shell fired almost 90 years before.

Such metal-infested fields in Belgium and northern France also produce each year, as tractors and ploughs become ever more powerful, a steady harvest of bones – British, Australian, Canadian, French and German.

The one part of the Great War battlefields where this does not happen is the ridge just to the east of Verdun where 300,000 French and German soldiers died from February to November 1916. When the war ended, the blood-soaked Verdun moonscape was not cleared and reclaimed for agriculture, as were the "British" battlefields of the Somme or Passchendaele. Instead, the entire battlefield was planted with conifers. The forests were meant to be a permanent memorial to help future generations remember.

In truth, the Verdun forests, now grown dark and tall, make it harder to recall what the battle was like. Around Ypres, for all the tides of time chronicled by Stefan Boness, it is still possible to grasp the contours of the three great Flanders battles of 1914-18.

Why are we still so possessed by the Great War? What makes so many British people go back to the Somme and Ypres? Why did Boness spend so long – several years – taking photographs of the battlefields (which have just been published as a book, Flanders Fields, Bildschöne Bücher, €35)?

Martin Middlebrook says that, for one reason or another, the 1914 war has "come out from behind the shadow" of the 1939 war. Why?

One of the forces at work is the new obsession of rootless, modern humanity with genealogy. Almost all of us have a relative who fought at the Somme or Ypres or Verdun. (My own great uncle Monty Lichtenstein, I discovered recently, died in the battle for Passchendaele.)

Another British historian, Michael Stedman, points to the recent explosion of writing about the Great War, in both history and fiction (an explosion begun by Middlebrook in the mid-1970s). Stedman believes that this surge in what he calls "people's history" – converted into successful fiction by Sebastian Faulks, Pat Barker and others – has taken the faded sepia memories of a thousand photographs of dead great uncles and restored them to full colour.

There is, however, something else. The Great War gnaws at our guts and our collective memories because it was the war that shaped the modern world. "The logic of Hiroshima began at Verdun," a French historian, Jean-Yves Le Naour, once told me. "It was at Verdun that the notion of industrialised mass destruction was pushed to its limit for the first time."

At Verdun, yes, but also within a few months, and even more destructively, on the Somme.

The 1914 war was the culmination of a 19th century that increased the productive – and destructive – power of mankind beyond the scope of dreams and nightmares. It was a war fought with murderous inventions, such as machine guns, high-powered artillery, poison gas, tanks, war planes and fragmentation shells.

More importantly, it was a war fought with new economic and logistical capacities, from railways, to tinned food and the industrial and taxation power of the modern state. Never before could armies of 3,000,000 men have faced each other in a space of 200 square miles for more than 10 months. To kill 1,200,000 people in one battle (the Somme) takes political organisation and determination, great administrative skills and economic power, not just military strength or callousness.

The 19th and early 20th century also saw immense advances in political rights, in education, in respect for the humanity of the common man. On the Western Front at least, the Great War was fought by educated men – one of the first generations to be universally educated. On both sides, it was fought – not universally, but largely – by men who had the vote.

The First World War was also the first in history in which systematic trouble was taken to give soldiers individual graves (at least those who could be found and identified). This collision between industrialised mass slaughter and democratic respect for the individual is the great paradox of the First World War, one of the reasons, maybe, why it haunts us to this day.

Why did a Western world that was beginning, for the first time, to respect and value individuals, pour them into its new mincing machine of military-industrial power? Why did an educated population stand for it?

The answer, in part, is that one of our first uses of mass literacy and mass education was to inculcate an unthinking patriotism and nationalism – in Britain and France, as much as in Germany. The innocence and confidence of the young men in boaters and flat caps who queued to join Pals' Battalions was born from a Boy's Own-magazine conviction that British Is Best, as much as a belief in freedom and democracy.

And yet can we confidently state that they were not defending freedom and democracy? Can we say for certain that the world would have been little different if German militarism had triumphed over flawed democracy in 1914?

Talking to visitors to the battlefield today, you still find those who believe that the war was a great patriotic and democratic crusade and those who believe that it was a criminal waste of life. You find many people who – quite reasonably – believe both.

Before he became a historian, Michael Stedman was a schoolteacher who took scores of school trips to Flanders and Picardy. "The 1914-18 war raises questions which go to the core of mankind's existence," he says. "Is it right to fight for what you believe in, even if you know that warfare, with modern weaponry, leads to unimaginable indignity and suffering?

"At what point do you stand up to evil? How can you distinguish good and evil from nationalistic ranting and posturing? All those questions remain with us. All are unresolved and perhaps will never be resolved ... I know from taking school parties to the Somme, that just going to the battlefields forces young people to grow up, to face these questions for the first time."

Here, then, is one reason why we should never bury the First World War. Even for the internet, mobile-phone, cheap-air-fares generation, this is not a quaint, distant event like Waterloo or Agincourt.

The optimistic lesson (if an optimistic lesson can be learnt) is that the Great War happened at a unique and unfortunate time in human history. It happened when military, and economic, power had achieved an extraordinary new zenith. It also happened before the power of the modern media, and the scepticism of modern minds (partly created by 1914-18) could hold politicians to account for wreaking such destruction.

Each British death in present-day Afghanistan is (rightly) the subject of anguish. Imagine the reaction in the age of instant news to the 29,000 British deaths on the first day of the battle of the Somme (largely in the first couple of hours) on 1 July 1916.

But what of the media jingoism that trumpeted the US and British invasion of Iraq? What of the persistent, intolerant, anti-Western and anti-semitic propaganda in parts of the Arab world and Iran? The Great War also remains a great warning. In the days of mass terrorism and nuclear proliferation, it is a reminder of what can happen when two causes collide, each armed with technologies of mass destruction and each driven by a blind faith in its own righteousness.