Richard Holmes, professor of Military and Security Studies at Cranfield University and presenter of an absorbing new six-part BBC2 series about the war, Western Front, has thought about this question more than most. "I'm haunted by the First World War, and I think we all are," he says. "It has marked our century. When there was the possibility of a ground war in Kosovo, I'm sure that the Somme and Passchendaele were somewhere deep in the folk memory."
But more than that, he continues, "we see the First World War as a sundering, as the end of an era. The old world of Ascot and Henley and village greens - which Siegfried Sassoon writes about so movingly in Memoirs of a Foxhunting Man - was blown apart in 1914. Things were never the same again. The First World War marked a peculiar transition from a society feeling confident about itself and its place in the world to a sense of quite extraordinary poignancy. All that we'd fought for had turned to ashes, and men wearing First World War medals were on the dole.
"Also, Britain mobilised one quarter of its adult male population for the First World War. It was the first time since the Saxon age that military service had been a unifying factor across the community. Military service touched everyone, and it still touches us today whenever we go to a village church or see a war memorial on a village or a line in our family-tree stopping abruptly in 1916."
Perhaps our strongest impression of the war, however, is of its sheer futility. There is a popular conception of remote and arrogant generals sending millions of young men to be pointlessly slaughtered. Holmes thinks this image may not be entirely fair - and pins its perpetuation on another television programme.
"The First World War has been hi-jacked by Blackadder Goes Forth. Now, I love Blackadder and had tears rolling down my cheeks at the end of the series. But I get upset that the sufferings of our grandfathers are reduced to the picture of the Army trying to move Haig's drinks cabinet a couple of inches nearer Berlin. Blackadder hits the truth, but never slap-bang in the middle."
For Holmes, the war was "not exclusively about stupid generals and politicians - although they played their part. It was not the conspiracy that many people think it was. It was merely a tragic accident - from which the British Army emerges in better shape than it has been given credit for. The man on the Clapham omnibus will remember the Battle of the Somme but forget that in the last 100 days of the war, even in the opinion of the French, the British Army were at the top of their game. There was value and dignity in the war. That deserves recognition."
The fact that it hasn't is because, according to Holmes, "we need someone to blame. We can't accept that something as awful as this could have happened without anyone being put in the dock for it. It's hard not to imagine the generals as creatures from another planet ordering our nice young boys off to get killed. In fact, 60 generals died. It was more dangerous being a general in the First World War than the Second."
Holmes found making Western Front a deeply moving experience. "When you walk down the trenches near Ypres, it's impossible not to feel a connection with the men who fought there. The idea of `the lost generation' is not a myth. The First World War pruned many shoots that would have bloomed across Europe.
"Private Snodgrass did not have a choice about fighting," Holmes concludes. "It's not something to remember with bombast, but with pride and dignity. We owe those soldiers a debt. It may sound mawkishly sentimental, but at the end of the series, I crouch beside the grave of the Unknown Soldier and say `the least of these men was as much a man as I am'."
`Western Front' begins on Thur at 7.30pm on BBC2