• VIDEO: Stop-Loss trailer

STOP-LOSS begins in the Iraqi city of Tikrit. Brandon King (Ryan Phillippe), a US staff sergeant, and his men are working at a checkpoint when they're fired on from a car that careers towards them before taking off into a warren of narrow streets and alleyways.

Giving chase, they're caught in a firefight. By the time it's over, several women and children have been killed, one of King's men is dead and another has appalling injuries. The young sergeant has lost faith in his judgment and we've witnessed the war in microcosm.

It's a galvanising start to this film from Kimberly Peirce ( Boys Don't Cry), who talked about her personal stake in the war when she was a guest of the Sydney Film Festival. Soon after the terrorist attacks of September 11, 2001, her 18-year-old brother enlisted in the army and, in 2003, he was sent to Iraq.

This got Peirce interested in doing a documentary about her country's soldiers and their experiences once they returned to home soil. And in the course of her research, she learnt about "stop-loss", a US Government policy that allows the army to force servicemen back into the war after their contracts have expired and they've made up their minds to resume civilian life.

King is just such a soldier. Sustained by the thought that he'll no longer have to wrestle with the need to put people's lives at risk, he sheepishly goes through the hero's welcome that greets him on his arrival in his Texas home town.

Then he discovers that he's about to be shipped out for yet another tour of duty. The military is exploiting a legal loophole permitting it to keep him fighting as long as they think he's needed. King's response is immediate. He curses the lieutenant-colonel who confirms the news and is promptly sentenced to a spell in the stockade. But before he's taken in, he punches one of his guards and goes absent without leave on a journey that introduces him to some of the thousands who share his predicament.

His adventures on the road, however, are only part of the story. Interwoven with them is an account of his unravelling relationship with his former buddies, none of whom can understand why he's taken such a radical step.

When still visualising the film as a documentary rather than a feature, Peirce came across digicam footage taken by the soldiers she interviewed and she's used the idea here to put audiences inside the lives of King and company.

The film is at its best when dwelling on the physicality of these young men - the quickness of their tempers, the speed at which their rough-housing can accelerate into a grudge match and their deep-seated conviction that it's them against the rest, the rest being those who've never been in combat. The two whom King has known from childhood are especially bitter about his defection, partly because they realise they were happier at war than they are at home. They view peacetime freedom as more a threat than a blessing.

Tommy (Joseph Gordon-Levitt) has quarrelled with his wife and is consoling himself with alcohol. Steve (Channing Tatum) has realised that he's not ready to settle down with Michelle (Abbie Cornish), the girl who waited for him for years - he's already married to the army. Wisely, Peirce doesn't try to soften the men's attitude to the people whose country they've been occupying. To Tommy and Steve, all Iraqis are "Hajis".

Their loyalty to one another is paramount and the Iraqi civilians who've died in their crossfire rarely enter their calculations. They're rough company and Phillippe, with his smooth golden looks, seems out of place alongside them.

Admittedly, King is meant to be the most thoughtful of the bunch. Unlike the others, he is haunted by the innocents whose lives have been ended in the battles he's fought. But Phillippe strains to give him the toughness and the gravitas he's supposed to exude. After all, the army wants to keep him because of his gifts as a leader.

When he goes on the run, planning to petition a senator he knows, Michelle goes with him, the excuse being that she regards him as a surrogate brother.

It's hardly the most credible of Peirce's plot devices but it gives Cornish a chance to shine as a girl who has finally had to admit the truth; that she must say goodbye to the future she's counted on since adolescence. Her performance starts with her walk - a no-nonsense stride with elbows stuck out. Watch her stroll into a bar and she seems more credible as a small-town Texan than Phillippe does.

Ultimately the script lets the film down. The run of postwar torments suffered by the returning soldiers is unrelenting, beginning after the celebratory party on their first night home. They drink too much, lash out at loved ones and fly into rages.

The climax of the night is a scene where Steve, in the grip of a flashback, digs himself a foxhole in his lawn and takes cover against imaginary fire.

All of these events would be convincing in isolation. Sandwiched together, they leave you too numb even to try suspending disbelief.

It leads to the sense that the script has been compiled rather than written, playing as if Peirce has drawn it up from a checklist of issues demanding to be covered. For none of it comes near to matching the tragedy and promise of that opening scene.