In the small village where I grew up in southern England lived an old man called Harold Gillies. He had been a famous surgeon during World War I. Though very much older, he was a friend of my father; they shared a love of watercolour and from time to time would paint together.

He lived in a house set back from the road. I'd go there sometimes with my father to see the paintings that he had propped up along the mantelpiece. The only one I remember was of his "dear wife" in her coffin. She looked pale and serene, and though I must have met her on earlier visits, I have no memory of her but this watercolour. My father tried to divert me, but I wasn't to be diverted, and the old man stood beside me and said something to the effect that when death comes in due time it is not to be feared. In due time: it was a concept I had not encountered before.

Sir Harold Gillies knew about due time, and he knew about deaths that come out of time and in ways that are full of fear and dread. It wasn't until decades later, though I dare say that my father had told me before, that I realised who Sir Harold Gillies was. Knighted in 1930 for his pioneering work as a surgeon during World War I, he is sometimes called the father of plastic surgery for the techniques he devised to repair the faces of soldiers coming back from the trenches.

In Britain alone, 60,500 men were wounded in the face or eyes. Noses, jaws, whole faces had to be reconstructed. It was Gillies who first used the technique of a "tubed pedicle", which took a flap of skin from the chest or arm and grafted it to the face without cutting the blood supply. That way the skin would grow in its new position with greatly reduced scarring when the pedicle was eventually cut. If you have the stomach for it, go online and find the before and after images of his reconstruction work. He called it "this strange new art", and felt he would need to be as much an artist as a surgeon if these wounded men, so many of them young, were to be accepted back into society and not shunned as the monsters they appeared.

With this month marking the centenary of the start of World War I, I've been thinking about this small shadow of a memory. It's curious that events a century old still have a kind of immediacy. My parents were born at the end of that war, my father in 1919 and my mother in 1924, which meant they were 20 and 15 respectively when the next war began, at the end of which I was born. Talk of war was part of the landscape of my childhood. Both my parents had served in World War II, as had the parents of all my friends.

One father had lost an arm, another was lame; my father was deaf in one ear from a shell that exploded too close. We were the lucky ones compared to the girls at school and my older cousins whose fathers had been killed and their mothers widowed. And behind that war, was that other, earlier war. As a child I heard the names of family members killed in Flanders, and in houses we visited were sepia photographs of fiances and husbands wearing the same kinds of uniform in which they'd died. Schoolteachers and great aunts, old ladies in the village, we knew, were unmarried because after that first war, there were not enough men. "Superfluous" women, they were called.

It's not surprising that my generation grew up to be anti-war and we shortened our skirts and grew our hair at the same age our grandmothers and our mothers had put on their uniforms. And now, a hundred years after the start of that savage war, every bookshop I go into has a display of war books. Among them are good books, I don't doubt that, but the overall effect of those cover images of trenches and helmets and young soldiers running into battle leaves me feeling out of kilter with the celebratory mood, the accounts of battles and manoeuvres, the military histories.

There aren't many left who lived through the Great War and can remember its immediate aftermath. My generation will be the last to remember it through our parents and the talk we heard as children that taught us that the cost of war was high, and that the suffering it caused was not separate from heroism, but often deeply entwined with it.

No one wrote better about this than Vera Brittain, who was 19 when the war began in 1914. She was due to take up a place at Oxford University that autumn to read English literature. Her beloved only brother, Edward, was 17, a prime age to enlist, which he did with his two best friends from school, including Roland Leighton, with whom the young Brittain was in love. For these soft-faced boys - barely yet men - there was no question but to enlist. A fellow student who survived the war later described the mood of "appalling jingoism" at the school, with the headmaster telling the boys that if a man couldn't serve his country he was better off dead.

When Brittain went up to Oxford, Edward, Leighton and their friend Victor Richardson went to training camp, full of patriotism and expecting the war to be short, sharp and heroic. It was none of those things. All three would be killed. "The three musketeers," Leighton's mother had called them.

After the fighting was over, but its grievous legacy was not, Vera Brittain wrote Testament of Youth, the brilliant memoir that was first published to controversy and acclaim in 1933. She wrote it, she said, because she felt that it was only by attempting "to write history in terms of personal life" that she "could rescue something that might be of value, some element of truth and hope and usefulness from the smashing up of my own youth by the War". And smashed up it was.

Leighton was the first to be killed. He and Brittain had become engaged while he was on leave in the summer of 1915. She was nursing by then, having given up her place at Somerville College, Oxford, much to the dismay of its formidable principal, Miss Emily Penrose. Many a woman could nurse at the front, was her argument; in the longer run, was it not more important to educate women into positions of power that might make war less likely? We might not be so impressed by this hope today, having lived to see women in positions of power, but for Miss Penrose's generation, the education of women had been hard fought - and was not yet won.

In 1914, when Brittain began at Oxford, women could study for a degree and take the exams, but it was not until after the war, in 1920, that they were allowed full degree status. Brittain understood Miss Penrose's argument - how could she not? - but the counter-pull was stronger: a kind of "accursed" generational duty to "listen and look" whether she wanted to or not, and share, in so far as she could, the fate of her male contemporaries.

Brittain spent the rest of the war nursing the wounded. By the time Leighton was expected back in England on leave at the end of 1915, she was at the 1st London General Hospital "amid the slums of Camberwell Green" in south-east London, where wounded soldiers were brought from France.

It was tough work, with long hours, bleak quarters and a daily fare of horrible wounds. The first dressing she had to attend to was a "gangrenous leg wound, slimy and green and scarlet, with the bone laid bare"; she saw orderlies faint at the sight of less. After months of this, when she heard Leighton would be in London for Christmas, she treated herself to a new dress - black taffeta with "scarlet and mauve flowers tucked into the waist" - and "dashed joyously" to the phone expecting to hear his voice. She learned instead of his death at a casualty clearing station in France. It was early on Christmas morning. She had just come off night duty.

Two months later, Leighton's kit was returned to his parents, including the uniform - "those poor remnants of patriotism" - he was wearing when his stomach took the impact of a machine gun. "I wondered," she wrote, "and I wonder still, why it was thought necessary to return such relics - the tunic torn back and front by the bullet, a khaki vest dark and stiff with blood, and a pair of blood-stained breeches slit open at the top by someone obviously in a violent hurry. Those gruesome rags made me realise, as I had never realised before, all that France really meant." She was glad neither Edward nor Victor were there to see them. "The horror of war without its glory," she wrote in a letter to her brother.

These days, Camberwell Green is no longer an area of slums - I have a niece who lives there, in a street lined with trees and window boxes - but in 1915, after Leighton's death, the drab mile and a half walk between the hospital and her quarters at Denmark Hill became unbearable to Brittain. She volunteered for an overseas posting, and was sent to France as a VAD (Voluntary Aid Detachment) nurse at the frontline 24th General Hospital near Étaples.

It was not an easy option; in some ways rather masochistic, she admits, as if suffering could only be born with more suffering.

At Étaples, within sound of the guns, she saw much worse: a third of the men were dying, and there was little that could be done to staunch unstoppable haemorrhages, and repair ruptured intestines spilling from the open gut of soldiers too badly wounded for the journey back to England. To her dismay, at least initially, she was "pitch-forked" into a ward for wounded German prisoners. There she confronted the truth that the enemy is also human. Her patients were stoic and appreciative; their bodies as vulnerable and tender as the bodies of the British soldiers she'd nursed in London.

When she found herself "with barely a second's hesitation" holding the hand of a young dying German, she found herself thinking "how ridiculous it was that I should be holding this man's hand in friendship when perhaps, only a week earlier, Edward up at Ypres had been doing his best to kill him".

Victor Richardson was the next of the three friends, those "three musketeers", to die. He was one of the 650,000 who took a bullet in the head. It went in behind the eye. He was also badly wounded in the arm. It was April 1917 during an attack on Arras. He was brought back to England, though not to the hospital where Harold Gillies was working. He lost the use of both eyes, and of the injured arm. The Military Cross was awarded to him a few weeks later but this tribute, Brittain reports, "did little more than intensify the stricken confusion into which his family had been plunged". When she heard of Richardson's wounds, she returned to England in time to spend a few days with this friend she'd known since she was a girl. "If I had not been looking for him I should not have known him," she writes of arriving at the hospital. He died in June of a cerebral haemorrhage. "So much human wreckage had passed through my hands, but this ... well, this was different."

Different again, and worse - if there is a hierarchy of horror - was Edward's death just five months before the war ended. He had been wounded in 1916 and had been shipped back to England to the 1st London General Hospital while Brittain was still there. Nursing on a different ward, she had had to dodge the sister on Edward's ward, who thought she was using her brother as an excuse to "flirt with the 20 other officers whose beds surrounded his". When he recovered, the irony was not lost on either of them that his survival meant his return to the front for another chance of being killed. She didn't go to Victoria Station to wave him goodbye, as she had done Roland, "superstitious" that such a farewell would again be the final one. During the next long, "rotten" year when more and more of their friends - ever-widening circles of friends of friends and relatives - were killed, she held on to the frail hope that Edward would survive, as he was the kind of person "who kept his head in a crisis". But on the morning of June 15, 1918, he was killed by a sniper while leading a counter-offensive against the Austrians in the mountains of northern Italy.

When Leighton and Richardson had been killed, letters came in from friends and commanding officers; there was almost too much detail. When Edward died, the reports were curiously vague; he was shot through the head, but quite how was not clear, and there were few friends left to write of him, and none whom Brittain also knew and could talk to frankly.

It wasn't until 1934, a year after the publication of Testament of Youth, that she was told by his commanding officer that days before he was killed, Edward had learnt he was being investigated for a homosexual relationship with a fellow officer - some letters had been intercepted - and was facing a court martial.

Had he shot himself in the head? Had he run ahead, inviting enemy fire? Or was it, as with hundreds of thousands of others, simply the fate of the soldier? Brittain would never know, but would live forever with the knowledge that he had not been able to confide in her, so great was the shame and the stigma of homosexuality in 1918, and that whatever the course of those last hours of Edward's life, they "could only have been bitter".

After the war, Brittain returned to Oxford and switched from English to history, in order, she said, to understand "how the whole calamity had happened" and how it had been possible that her generation, "through our own ignorance and others' ingenuity", were "used, hypnotised and slaughtered". She graduated among the first women to be granted full degree status and remained a pacifist for the rest of her life, writing passionately against war, including during World War II. She was not deterred by the dog shit posted through her letterbox and continued on after the war in equally passionate support of nuclear disarmament.

Brittain's daughter, the English Labour peer Shirley Williams, who was born in 1930, remembers her mother late in life, standing firm at peace rallies, blocking the road until led "gently" away by the police. When Brittain died in 1970, her ashes were scattered on Edward's grave in Italy.