Johnny Mad Dog, Jean-Stéphane Sauvaire, 97 mins, (15)

Many films can justifiably claim to have captured the horror of war, but few have done so as mesmerisingly as Johnny Mad Dog.

What's more, this French-made film evokes not just war's horror, but also its chaotic, vicious and fundamentally infantile nature. Shot in Liberia, Jean-Stéphane Sauvaire's feature is not specifically about the civil wars that plagued that nation until 2003, but it draws directly on them and on conflicts in Rwanda, Sierra Leone and elsewhere. Based on a novel by Congolese writer Emmanuel Dongala, the film shows how war both destroys youth and deploys it as a tool of destruction. Johnny Mad Dog is about child soldiers: boys in their teens, and younger, recruited into the militia and used to implement violence, sometimes of an intensity at which adult soldiers might baulk.

At the very start, the film pitches us directly into confusion and frenzy so abruptly that we barely have time to register what's happening. Amid a cacophony of terrified screams and furious yelling, we begin to gather from the staccato-edited images that we're witnessing a raid by a squad of armed boys. They serve a rebel faction and patrol their (never specified) country, terrorising innocents, looting, raping and dispensing summary executions on a whim. Their adult general Never Die (Joseph Duo) whips up their frenzy with battle chants and other bonding rituals that seem more akin to brainwashing; on the verge of battle, he hypes them up further by cutting their foreheads and rubbing cocaine into the wounds.

The narrative follows the boys' slaughterous advance on a city where they intend to drive out the government and anyone associated with the Dogo people they have been taught to hate. Foremost among the boys is Johnny (Christopher Minie), a blank-eyed but ferocious youth, totally committed to his faction's cause. But he is by no means its most fearsome member: more alarming still is a manic squirt, aged possibly 10 or 12, No Good Advice (Dagbeh Tweh), whose murderous impulses are untrammelled by the thoughtfulness that has come to Johnny with adolescence.

Meanwhile, the boys' career of violence is intercut with the travails of Laokole (Daisy Victoria Vandy), a young girl attempting to protect her young brother and her legless father. While she represents the hope for compassion in a world gone mad, she is never sentimentalised. When she and Johnny have a brief, tense encounter early on, we know that their paths are bound to cross again. Anyone expecting the outcome to be a reassuring and cathartic redemption, be warned – it's not that kind of film.

In a quiet moment, Johnny confesses that he has been a fighter since the age of 10, and can't remember his parents or his real name. All these children have lost their pasts, and now have only wartime names: Jungle Rocket, Nasty Plastic. Caught between childhood and the vicious parody of adulthood that has been foisted on them, these boys take the Lord of the Flies condition to an extreme. Their insults, barked in English and in West African dialect, often have an edge of playground insult – "Are you a fucking crybaby?" – as if these are school bullies who have been armed and granted absolute licence. But sometimes we are reminded that they are still children. Facing UN guards, their bravado takes on an edge of sulky caution; commanded to surrender the live pig he has confiscated, No Good Advice turns fretful, as if about to lose a teddy bear.

Together with its harsh edge of quasi-documentary realism, Johnny Mad Dog is also potently stylised: Sauvaire variously musters a kinetic nerviness close to French cinema's arch-extremist Gaspar Noé and establishes a tone of eerie nightmare reminiscent of vintage Herzog. Especially unsettling is the young soldiers' chosen kit: their wigs, angel wings and football helmets seem to have come out of the nursery dressing-up box. Early on, a muscular youth calmly steps away from a scene of violence to try on a wedding dress: the image adds an incongruous dash of brutal homoeroticism à la Jean Genet. All this might seem like Sauvaire's wayward invention, but the film closes with documentary photos of Liberian soldier boys and there it all is, frou-frous, fright wigs and all.

Remarkably, the film was made with the support of the Liberian government, although you'd think that it promoted the worst conceivable image of the country. Some might object to a European-made film depicting an African nation as a living inferno and young Africans as feral killers. Yet, compare the film to well-meaning commercial fictions about African conflict – Shooting Dogs or Hotel Rwanda – and you can see that Johnny Mad Dog makes no concession to European or American perspectives. There are no white characters to keep the viewer company, no reassuring representatives of official sanity. Made with an African cast, and representing a wholly African experience, the film immerses us without protection in its world, rather than drop the subject cleanly packaged into our laps.

Sauvaire's cast actually contains several boys who themselves fought as soldiers, even participated in horrors like those represented. But you don't come away with the sense that the boys are being exploited for shock value, or even for gratuitous authenticity: there's a sense of ritual re-enactment at work here, a sort of community-project Theatre of Cruelty. Confrontationally intense, Johnny Mad Dog is not only one of the most disturbing war films I've seen; it is surely among the most troubling films ever made about childhood and its exploitation. Equally engaged – and furiously so – in contemporary reality and in the power of images, Johnny Mad Dog is incendiary stuff, extreme cinema for an extreme world.