Liberian Girl (Royal Court)

In order to escape that which would kill her, Martha has to become part of it. Her aunt, Mamie Esther (Cecilia Noble), cuts off her hair and straps down her chest, dresses her in jeans and a T-shirt, before nudging her to join the rebel army that has ransacked their village. Martha is 14 years old.

Diana Nneka Atuona's debut play, written after a Royal Court writers' course in Peckham, is quite something: a rampage through Liberia alongside a band of child soldiers, as the country falls foul to a bitter civil war. We see teens trained up like attack dogs, fired up with drugs and hollow politics they don't fully grasp, and taking the spoils of war for themselves; sometimes possessions left behind, sometimes women – girls.

Even so, everything retains a glimmer of innocence, an edge of the playground: impromptu kickabouts, dressing up, smokes behind bike sheds. Training looks like a PE lesson – just with AK-47s. Boys adopt puffed-up nicknames: Killer, Double Trouble and, for Martha, Frisky. It's like they have to leave their old selves behind.

In Matthew Dunster's wraparound staging, all this happens within touching distance. It's all the more horrific for that; almost too intense to bear, too uncomfortable for entertainment. We could intervene. We could defy the actors that shunt us, forcefully and at gunpoint, around the space, separating men and women as they do. No-one does.

"The ensemble acting is absolutely first-rate"

Complicity is central. Ours, of course, both in the room and in the world, but Martha's too. One extraordinary scene – Atunoa's best – sees her comrades arguing over two terrified teenage girls, one prettier than the other. Unable to agree, the boys offer their victim her choice of rapist. Later, Martha's pushed forwards for a go. In simulating the act – even though it's her only option, though her survival depends on it – she betrays her sex. In time, her sex will betray her: her womanhood starts with blood on the battlefield. It's a stunning, startling moment.

Atunoa's play is full of them, mixing a mythic quality with the godawful realities of war. It has all the sweep and horror of a Brecht epic, only Liberian Girl never feels like a fable. Dunster doesn't let it: his quickfire, brutal staging is totally transporting. Anna Fieschle's design combines real materials – metal, sticks and concrete – with smart signs of its own theatricality, and the ensemble acting is absolutely first-rate: utterly, awfully convincing. Atuona's writing, too, never tries to be too clever. She let's her story stand for itself.

As Martha, Juma Sharksh finds a kind of serenity in the mayhem. Her soldier wears a mask, going through the motions to avoid detection, but without the frenzied relish of her peers. Valentine Olukoga is all hot breath and adrenaline as Killer, while Michael Ajao makes a doltish Double Trouble. For all their cruelty, though, they're still children – some "so small they can't even carry their guns." How can they possibly be to blame?

Liberian Girl plays at the Royal Court until 31 January 2015, before touring to the Bussey Building, Peckham (3-7 February) and the Bernie Grant Arts Centre, Tottenham (10-14 February) – for more info and tickets click here