Lance Corporal Joshua Bernard lies on his side in the Afghan earth, his gun still clutched in his hand. The air is speckled with the dust thrown up by the rocket-propelled grenade that has just been fired from a grove of pomegranate trees, blowing off one of Bernard’s legs.

As the camera shutter clicks, two other US Marines, blurred in their frantic efforts to save his life, are shouting: “Bernard, you’re doing fine. You’re gonna make it.”

The 21-year-old soldier did not make it.

This photograph of the dying Marine, taken by the Associated Press photographer Julie Jacobson on August 14, moments after a Taleban ambush outside the village of Dahaneh, has provoked fury in America. Robert Gates, the Defence Secretary, accused AP of a “lack of compassion and common sense” in distributing the image to newspapers: some of these published it, while others chose not to.

Corporal Bernard was the 19th American soldier to die in Helmand in a single month. American public support for the war in Afghanistan is waning fast, and Congress is expected to resist calls from Barack Obama for more troops. Defenders of the photograph accused Mr Gates of attempting to sanitise the conflict, by concealing the grim reality of the war at a critical political juncture.

The photograph violates one of the oldest taboos, by intruding into the sacred privacy associated with the moment of death. It was published in defiance of the wishes of the dead man’s family and it raises uncomfortable questions about the dividing line between voyeurism and reportage.

Yet the image is so frank and immediate, so intimately revelatory, that all other considerations are secondary to its raw impact. It is not a gory photograph. It does not, as too many war photographs do, turn tragedy into leering pornography. The bloodshed is blurred, the face out of focus. The soldiers are almost anonymous, performers in a battlefield tableau.

There is something almost mundane about Bernard’s death, on a dusty bank in a foreign field — a single frozen moment in which the nature of the war itself, in both its heroism and horror, seems to be localised and symbolised.

Very few images of war achieve this, although when they do, their power to shape our collective consciousness is immense. Knowing this, the authorities that wage war have always sought to control the public image of warfare.

An odd parallel to the Bernard photograph is the famous Victorian painting Remnants of an Army by Elizabeth Butler, depicting William Brydon, survivor from the 1842 retreat from Kabul, in which 16,000 men, women and children were massacred by Afghan tribesmen. The picture shows Brydon, grievously wounded, weaving into Jalalabad on a dying horse.

The painting was immediately controversial. Instead of depicting the British as victorious (happy and glorious), it showed an army defeated and desolate, with a single, half-dead survivor. The painter was unrepentant: “I never painted for the glory of war, but to portray its pathos and heroism,” she wrote.

When Roger Fenton, one of the founders of the Royal Photographic Society, was sent to cover the Crimean War in 1855, he went with strict instructions not to photograph any dead bodies. Photography in the First World War was subject to intense censorship. Only four years ago did the Pentagon finally lift the official ban on photographing military coffins.

This desire to control the imagery of war reflects the capacity of photography to convey the blunt truth about conflict in a way that no other art form, including the written word, can achieve. At Etaples, in 1917, Wilfred Owen wondered how anyone would be able to visualise “the very strange look” on the faces of men before battle on the Western Front: “An incomprehensible look ... more terrible than terror, for it was a blindfold look, without expression, like a dead rabbit’s. It will never be painted.”

But it could be photographed. Half a century later, Don McCullin captured that bleak emptiness on the face of a shell-shocked GI at Hue in Vietnam.

A single, stark photograph like this can encapsulate an entire war. Nick Ut’s 1972 picture of a nine-year-old girl fleeing her village after a napalm attack represents, for many, not just a snapshot of the Vietnam War but the moral complexity of the war itself.

Robert Capa’s photograph The Fallen Soldier similarly evokes the Spanish Civil War: it is irrelevant who the soldier was, and where he had fallen; nor does it matter that the photograph may have been faked, for its power lies in not in the literal evidence it offers, but the greater meaning it conveys.

Photography can blunt emotional reaction, as well as stimulate it. With the glut of war images pouring out of Iraq and Afghanistan, it is easy to become inured to their impact.

Jacobson’s photograph of the dying Marine is one of very few, from either war, to merit the overworked adjective “iconic”, in the original sense of an object of veneration, a physical article freighted with moral meaning. The picture’s power lies less in its shock value than its demand for a response: for some it shows the courage of war, for others it is a condemnation of that war, but more important than either reaction is its depiction of what war really is, in its daily, dirty, arbitrary violence.

Susan Sontag wrote of the unique power of the photograph to jolt the conscience out of the torpor that comes with visual overload. “A photograph can’t coerce,” she wrote. “It won’t do the moral work for us. But it can start us on the way.”

The ambush at Dahaneh is already a footnote. Joshua Bernard’s name will be forgotten, but his influence on history is likely to be profound and lasting.

The image of his last moments does not demean his death, but immortalises it.

Times Recommends

• Strip away the figleaf and reveal naysayers
• Dave’n’George: there may be trouble ahead
• I do not accept that £64,000 a year for politicians is peanuts