Two exhibitions, two anniversaries. The first, In Memoriam at the Imperial War Museum, commemorates 90 years since the end of the war to end all wars. The second, On the Subject of War at the Barbican, marks seven years of the "war on terror", a war, by definition, without end. The first places us on familiar poppy-strewn emotional territory, but retains its inviolate capacity to move and to shock. The second puts us squarely in no-man's-land.
The thing about the great artists of the First World War was that they knew exactly where they stood, and where their audience stood. They were muddied near the front lines at the Somme, or stranded on the beaches of the Dardanelles, and they were resolutely on the side of the ordinary soldier, the primary victim of the war. Paul Nash could delineate the suffering he experienced first-hand at Ypres and give it universal significance; John Singer Sargent, cajoled into service as a war artist by a letter from Lloyd George, reproduced here, could frame his epic painting of the blindfolded leading the blind in Gassed (1918-19) and be sure where the viewer's sympathies lay; William Orpen could drape a Union Jack over the tomb of the unknown soldier in To the Unknown British Soldier Killed in France (1922-7), and invest it with all the charged ambiguities of sacrificial patriotism.
There is an extraordinary photograph included in In Memoriam which shows the 137th (Staffordshire) Brigade being congratulated by General Campbell for their capture of the bridge at St Quentin Canal. The men, thousands of them, clothed in mud, are ranged against the far bank of the canal, like the toy soldiers of a Chapman brothers hell, and seem as if they have been modelled from the hillside. Look closely, though, and each man seems to have a face made of stories, full of engagement in the desperate events that have shaped him. We can project this because we know it to be true. Each one of the thousands of survivors looks as full of life and as broken with sorrow as the last remaining survivor, the wonderful Harry Patch, now 110, who has never forgotten his own private war: "The day I lost my pals, 22 September 1917, that is my Remembrance Day," he recalls here, "not Armistice Day. Ninety years after and I always remember it, I never forget the three I lost."
What will the Harry Patches of the current conflicts in Iraq and Afghanistan say of their war at the end of this century? How will those waking up, as their new president says, in the deserts of Iraq and the mountains of Afghanistan, some of whom will not make it home, be remembered? What should we project on to their photographed faces?
Artists have struggled to get to grips with the last seven years of war, not least because they have looked in vain for a human scale to its horrors and have lacked an audience certain of its sympathies. Some have made powerful local images of skewed patriotism and forgetting - Steve McQueen's postage-stamp portraits of the British servicemen and women who have died come to mind - or claimed wider points about the people's sense of the illegality of war - Mark Wallinger's Turner Prize-winning recreation of Brian Haw's Parliament Square protest. But few have come remotely close to expressing the texture of the conflict itself in the way that Nash and Sargent's bold lines conveyed the blood and mud of that earlier sacrifice.
The four artists grouped in the Barbican show - the most rigorous attempt to address that absence I have seen - take on those difficulties squarely. They are preoccupied with questions of vantage, of where the front lines of the current war might lie; and of empathy, of how to engage populations in wars conducted in their name when simple lines of propaganda will not do. Each of the artists has an ambiguous relation to the war itself. The photographer An-My ê was evacuated from Saigon in 1975 and went on to study medicine in America before picking up a camera. Omer Fast is an Israeli film-maker, born in 1972; Geert van Kesteren is a Dutch photojournalist, who works on assignment for the German weekly Stern; Paul Chan is a New Yorker, a satirist and a campaigner.
They come at the war from different places. An-My ê was there at the beginning. She went with American troops in 2003 to 29 Palms, a desert centre in California where marines train for combat in Afghanistan and Iraq. Two films she made there capture both the inadequacy and the surreal nature of that preparation. In one, her camera rests quietly on the young faces of the American soldiers, men and women in shades, half listening while they are briefed about the far better thing that they should do. Unlike the faces of the soldiers at St Quentin, theirs seem absent of stories. The second film, running concurrently, shows those same men and women set against the John Ford desert landscape, barely visible in the vastness, running in minuscule batallions, attacking forces that are figments of their commanders' imaginations.
An-My ê, whose name sounds like a massacre, follows her troops on to their aircraft carriers in Iraq, where they are protecting oil refineries, her studiedly beautiful large-format prints now revealing how they have never stopped playing games - a helicopter apparently lands on the lip of the horizon; a machine gun is primed beside a deck chair facing the empty blue ocean; marines inadvertently line themselves up before human-shaped targets on board the ship. They are honest images of a war based on a fiction. The soldiers may have been posted to fight, to put their desert training into practice, but as Gertrude Stein observed in another context, having arrived, there is no "there" there.
Something of that badly scripted atmosphere is also present in Omer Fast's extraordinary film The Casting, which was the star of this year's Whitney Biennial, but the emotion of it is far more mangled. Fast toys with the way that history is mediated - in the past he made a memorable film with the Polish extras on Schindler's List, which mixed their experiences of Hollywood with their real memories of war. Here he brings that fascination into sharper focus.
The Casting is based on an interview Fast made with a sergeant in the US army just back from Iraq. The sergeant told him two stories: the first involved a disturbing one-night stand he had with a woman while he was posted in Germany, the second was a recollection of an incident that occurred when on patrol in Baghdad. On one side of a double screen the interview with the sergeant is played back but the two stories are spliced together - a terrifying high-speed journey home with the German woman and her des ire for him to "hurt her" are intercut with his narrative of Iraq, in which, in trying to stop a car at a roadblock, he ends up inadvertently shooting and killing one of the car's passengers. On the other side of the screen a separate film runs, which is a dramatisation of these nightmares played by actors.
The effect of Fast's careful manipulation of what is a chilling, awful account of a senseless wartime murder is mesmerising. The sergeant clearly does not know what to do with the stuff that is in his head, is not sure how real it all is, and Fast recreates that blurring. The soldier's account of the date with the girl, "it started off like real nice and ended horribly", also stands for his sense that his mistake in shooting the Iraqi man should have had some consequence: "I didn't know at the time that kind of thing would be just ignored," he says. Or, perhaps most wretch edly: "That was probably one of the worst days."
Those worst days never seem to end in Geert van Kesteren's photographs, mostly taken from his landmark book about Iraq, Why Mister, Why?. But, like Fast, he allows human complexity into his record, the complexity that will enable sympathy. For a time van Kesteren was embedded with an American troop and he captures both the hi-tech inhumanity and humiliation of their raids on civilian houses - the world as seen through a night-vision rifle sight - as well as the impossible loneliness of their own mission. Later he photographed in Baghdad without military protection, and his images of the mundane depredations, incongruous joy and sudden terror of family lives have the power of letters home from the trenches.
It is the semblance of that kind of power that also draws Paul Chan to the conflict. In Re: The Operation, a film that bridges the gap between presidential rhetoric and reality, he imagines the perpetrators of the war writing anxious letters home from the front line, in horrible parody of all those SWALK ('sealed with a loving kiss') hopes and prayers. Karl Rove addresses his wife from his distant bunk in some unknown desert: "I am useless here. It's the same old story. He listens but he doesn't. Forget about Dick: it is as if I don't exist. Condi is pushing for a pull back and Donald wants to enlarge the field. Colin is in his own world using a kind of surrealist diplomacy that scares everyone." Private Rumsfeld, meanwhile, muses bleakly that "we must learn to live with low-density hope". There is some corner of a foreign field that is forever neocon.
"In Memoriam" is at the Imperial War Museum, London SE1, until 6 September 2009. http://www.iwm.org.uk
"On the Subject of War" is at the Barbican Art Gallery, London EC2, until 25 January 2009. http://www.barbican.org.uk