Fictional films set in Iraq or Afghanistan have had a notoriously unfavourable commercial reception. Fortunately that hasn't deterred documentary makers from exploring the same territory, the budgets being so much smaller, the box-office expectations modest to negligible.

There's another reason why documentary is better suited to the subject: its immediacy, not just in visual terms, but in its capacity to reach the screen more quickly without the obstacle course of studio schedules and test screenings to negotiate. It would be foolhardy for any filmmaker to aim for a definitive portrait of an ongoing conflict, but the documentary form doesn't make the same promises of completion or containment that fiction does; we accept it more readily as a snapshot grabbed on the hoof.

The riveting new documentary Armadillo is an instructive example of a film which tries to have it both ways -- to evoke the unpredictability, anxiety and essential shapelessness of its subjects' lives but also to fashion the material into a rounded narrative more typical of a scripted project. Armadillo is named after a military base in Helmand Province that is home to 170 Danish and British soldiers. The director Janus Metz secured extraordinary access to the Danes on the base, and begins his film with a brace of scenes promising two different kinds of behind-the-scenes candour.

In one, a downbeat dinner-table conversation between Mads, a soldier about to leave for Helmand, and his family, who can't quite come to terms with what he's doing, establishes the film's intimate emotional texture. The second, showing Mads and his fellow recruits mauling a stripper during a raucous party the night before they leave, hints that the coverage will be no-holds-barred, no punches pulled. A deeply unsettling scene late in the movie, when soldiers strip Taliban casualties of their weapons in the aftermath of a ferocious battle and drag their bodies around while likening them to dead animals, confirms this for all time.

There has already been controversy surrounding the troops' behaviour in the film, particularly their shooting of injured Taliban fighters, which led to an inquiry in Denmark. But from a cinematic point of view, the picture is rather caught in a cleft stick. It wants the pell-mell, sand-in-the-eyes authenticity of reportage, which it achieves with its terrifying battlefield sequences, but it seeks also to frame that material within the reassuring arc of fiction cinema. The problem is that the latter can only compromise realism. Audiences are so alert to the significance of apparently trifling elisions and distortions in documentary that the tiniest hint of fraudulence or manipulation will unravel a lot of hard work.

I'm not casting any aspersions on Metz's motives, which I'm sure were beyond reproach. He has said:

The mission was to bring the war on Afghanistan back into people's living rooms and make them engaged. There was a feeling that nobody was really caring that there was a war in Afghanistan.

What better way to do that than to give documentary footage the viewer-friendly shape and rhythm of a movie? If only one form didn't risk cancelling out the other. I guess Metz made his job easier by following one group of men during one tour of duty. There's a narrative right there: some will make it home, others won't; even those that do will have experience etched into them. And as one of the soldiers who is about to leave when Mads and friends arrive tells the newcomers: "You won't be bored. You'll see action." That's Metz's promise to us too.

I just wish the film wasn't so tidy; it throws up so many questions that the neat structure seems inherently to disavow. It's giving nothing away about the body of the film to say that the final shot is a close-up of a soldier standing in the shower, his head bowed toward us as the water streams down his face. We are clearly meant to infer from this shot that he is damaged, seeking solace, cleansing himself, washing off the sins of combat. But he might equally be thinking: "Golly, this shower is refreshing. I do like a nice shower. Showers are so much better than baths."

I see that shot, stolen from a moment even more private than a family pow-wow or a macho shindig, and I think instantly of the director on the other side of the camera, negotiating with the subject to film him during his ablutions. Armadillo is a strong and disturbing piece of work. A small moment like this, though, can be enormously telling. Introduce an aesthetic contrivance into a documentary that purports to be gritty and you've given the audience licence to doubt.

"Armadillo" is released on Friday