Thomas Sutcliffe: The first casualty of war - and art

"I think it's a crappy picture myself," says an army publicist, as he welcomes three of the soldiers who raised the flag over Iwo Jima back to the States. "You can't even see your faces." The line is fictional - a sour counterpoint to the mood of patriotic boosterism which the picture has stirred back home, and which is the subject of Clint Eastwood's film Flags of Our Fathers.

And it's easy to imagine the photograph that this unimaginative flak thinks would be preferable - every soldier face front and beaming, the flagpole victoriously upright, arms draped around shoulders in an image of comradely togetherness. He's out on his own, of course, because virtually everybody else in America - hundreds of newspaper editors, millions of readers, the President and the jurors of the Pulitzer Prize for news photography - knows that this is a truly exceptional picture, one of those images that don't just sell a newspaper but an idea as well.

That's now a given about the snapshot that Joe Rosenthal took on the summit of Mount Suribachi four days after Marines had first landed on the beaches of Iwo Jima. And I write snapshot because that's what it was - Rosenthal had been building himself a plinth out of rocks to get a better angle on the flag-raising when he realised that he might actually miss the moment itself. By his own account, he swung his camera up and pressed the shutter without looking through the viewfinder, only later discovering that he had captured a near-perfect composition. The qualification is crucial, I think - because the potency of Rosenthal's image is due to its quite accidental mixture of aesthetics and artlessness.

One of its secrets is exactly the deficiency pointed out by Eastwood's onscreen cynic. You can't see their faces. Although the mother of the soldier at the far right of the picture famously recognised her son the instant she saw the photograph, for virtually everyone else it was an image of unknown soldiers, indifferent to the camera and released from the particular into a fine abstraction. There are other lucky breaks too: the fact that the angle of the thigh of the front soldier exactly mirrors the backward tilt of the flagpole itself; the fact that the fluid, wind-whipped wedge of the flag echoes the solid wedge of men beneath it - and that both triangles point to the right, which (because of the way we read print) we have learnt to associate with notions of the future and forward advance. Imagine the wind blowing in the other direction that day and you can virtually see potency leaching out of the picture.

Most serendipitous of all is the fact that the three figures at the rear (there are five in all but only three are clearly visible) create a stop-motion frieze which we subliminally animate into a single soldier driving forward - an effect that amplifies the crucial sense that this is an image of a task not yet completed.

Part of the photograph's power - and one reason for its profound effect on a war-weary America - is its implicit promise that one last heave will complete the job. As Eastwood's film reminds us, the fundraising poster which used the picture carried the caption "Now... All Together".

All these elements are precisely what a commemorative sculptor might have calculated - a set of well-established compositional dynamics. But if you want to see what's really great about the picture you have to look at what happened to it when it actually was turned into sculpture. When Felix de Weldon got the commission to create a Marine Corps War Memorial in 1950, Rosenthal's picture was an obvious starting point. But there were some compromises with the truth. The soldier at the front was moved backwards, to slot more neatly among his fellow soldiers, and the figure at the very back was repositioned - so that his body adopted a more dramatic forward tilt. Most significantly, de Weldon tidied away one of the more poignantly awkward details of the original image - the way in which the soldier at the back can't quite reach the pole.

We can't tell whether it's just been raised beyond his grasp or whether there wasn't enough space for him to get his hands to it in the first place - but in the Marine Corps memorial that tantalising gap has been closed. And with it goes a crucial element of the photograph - a human detail of falling short that rescues all those happy accidents from collapsing into mere patriotic bombast. I don't think it's entirely fanciful to suggest that it is at the core of the picture's impact, generating a tension that enlists every viewer in the cause (how can you not reach out with him?). Without it the picture would be pure art - and much the worse for it.