Frank McGuinness made his name with this First World War drama when it first played at this theatre in 1986. Today, it remains an oblique but remarkably powerful depiction of eight young Ulstermen facing doom at the Somme in 1916.

It’s a story so heavy with history that it could easily refuse to fly: the Somme, one of the great fiascos of military history, has a particular place in Irish Protestant memory. The 36th Ulster Division, in which the men serve, lost more than a third of its men in the first few days. But McGuinness, whose background as an Irish Catholic caused comment when the play first appeared, keeps the focus tight. There are no officers, no women, no Germans, no marching. We just get eight men serving King and country while Ireland is in turmoil. Who to hate more, the Hun or the Taigs?

But John Dove’s revival spends its first half taxiing on the runway. The soldiers meet up in their barracks. It’s edgy: they’re supposed to act like strangers, and indeed they do. But there’s an awkwardness about the way that Richard Dormer’s perversely playful Pyper rubs up against his fellow Ulstermen: the troubled preacher, the blowhards from Belfast. It’s a slow set-up.

And then, what do you know, the second half achieves the sort of poetical take-off that you go to the theatre to find. The characters go beyond bluster as they talk, in pairs, while on leave before battle. All of them are testing their ideas of heroism and masculinity: whether it’s Mark Holgate’s McIlwaine, banging his big bass drum till his hand bleeds, or Michael Legge’s Crawford, railing against the religiosity of Billy Carter’s Roulston. And then Michael Taylor’s spare set, dominated first by a red hand symbol, then by a rope bridge, turns with horrible smoothness into a trench.

McGuinness’s writing blends realism and expressionism, dream and dread. As the boys recreate the Battle of the Boyne, and summon up memories of the home they know they won’t see again, the tone is sad, savage, yet funny. The acting is excellent, especially from Dormer, who embodies all of Pyper’s contradictions — melancholic, lover, provocateur, lunatic — without making the mistake of trying to resolve them.

From slow beginnings, this Hampstead production goes on to grab the imagination with extraordinary skill. Explanations, blame? McGuinness leaves that to others. Here, we get the unforced, vivid horror of real lives being snuffed out.

Box office: 020-7722 9301, to July 18

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