Combat fatigue: Tim Hetherington's intimate portraits of US soldiers at rest reveal the other side of Afghanistan

Heads curling into arms, mouths lolling open, bathed in pools of light, the members of a 15-strong platoon of US paratroopers embrace some much-needed sleep. For them, war veers between extremes. Combat follows boredom follows combat. When they aren't absorbed with killing the enemy, they're busy killing time.

Their sleeping beauty belies the danger of their situation. In a tiny outpost built on a steep hillside in north-eastern Afghanistan's Korengal Valley, these men could be attacked at any moment. US troops nickname the Korengal the "Valley of Death" because, by the end of September 2007, one-fifth of all the country's fighting had happened here. In early 2008, Battle Company's Second Platoon, part of the Second Battalion of the US army's 503rd Infantry Regiment, built "Outpost Restrepo" to draw enemy fire away from soldiers based at the bottom of the valley. The fortification was named after Juan Restrepo, their platoon medic, killed during the first two months of their deployment.

For 15 months, between 2007 and 2008, the Vanity Fair photographer, Tim Hetherington, and reporter, Sebastian Junger, shadowed these men. The journalists compiled their work in a documentary, Restrepo, and a book of photo-graphy, Infidel, both released next month, illustrating how they shared food, patrols and sleeping quarters with the soldiers. The longer the reporters stayed, the deeper their perspective became. They learnt that these baby-faced slumberers could also weep over lost colleagues or explain to local villagers why they'd wounded their children.

"The book and film are about the intimacy of war," explains Hetherington. "And that's what I see when I see the photographs of these guys sleeping. We are used to seeing soldiers as cardboard cut-outs. We dehumanise them, but war is a very intimate act. All of those soldiers would die for each other. We're not talking about friendship. We're talking about brotherhood."

From their pre-deployment japes to combat trauma, this bond continually manifests itself in Hetherington and Junger's imagery. By the end of Restrepo, viewers will feel an affinity towards Second Platoon's characters: Specialist Misha Pemble-Belkin, whose parents didn't let him play with guns as a child and who likes drawing; Private Restrepo, an amateur flamenco guitarist who can stitch wounds; Sergeant Brendan O'Byrne, who thinks "God hates him" because of the horrors he has witnessed. Specialist Miguel Cortez is so traumatised by his tour in Afghanistan that he remembers "not caring about getting killed". During downtime, the men play poker, guitar, wrestle. When not engaged in conflict, Junger says they "never got to release the tension that built up from maintaining a constant state of readiness, and they prayed for contact like farmers pray for rain."

Hetherington's photography captures the intensity of these caged men, surrounded by the paraphernalia of war. Often, they are portrayed as tattooed, muscular, nihilistic warriors. Many have the word "Infidel" tattooed across their chests – the epithet the troops learnt (through intercepted radio communication) that the Taliban had applied to them. In repose, the US soldiers seem angelic, contemplative, or vulnerable. To remember their homes, they rely on pornography, sporting magazines and the words "9/11", "NY" and "For Mom" written on the ends of bullets.

"Tim saw things very differently from the way I did; he wasn't looking for dynamism so much as for beauty or strangeness or even ugliness," writes Junger in Infidel. "There were pin-up girls, a lot of fly-strips and a lot of ammunition. Sometimes those three things converged on a bedpost in ways that were easy to overlook."

The photographer, however, is more modest. "You can get bored of taking pictures of fighting," he says. "I got more interested in the relationship between the soldiers. That's where the shots of them sleeping came from. If you go to these places you can sometimes get all your media oxygen sucked up by the fighting; we were lucky to have time to explore other things."

The photojournalist adds that the "warts and all" realism of his work has attracted brickbats from America's hard Left, who wanted a wholesale dismissal of the conflict. But the objective truth of his lens is somehow more truthful than any verbal polemic.

"In America, soldiers are used by the right wing as a symbol of patriotic duty, but the truth is they are all individuals," he concludes. "And the Left want a moral condemnation of the war. What I say is that if we have a full understanding of what the soldiers can and can't do out there, it is a good starting point for peace-building. The heart of the war machine is in fact taking a group of young men and putting them on the side of a mountain. We need to understand that experience. Certainly if we have any hope of properly reintegrating them into society." 'Infidel' will be on show at Host Gallery, London EC1 from 20 September to 15 October, hostgallery.co.uk; the book will be published by Chris Boot on 7 October, priced £25. The documentary, 'Restrepo', goes on general release in the UK on 8 October