Gregory Burke’s Black Watch, directed by John Tiffany for the new National Theatre of Scotland, first appeared at the Edinburgh Fringe in 2006, to great acclaim. It began a world tour in 2007, and now finally makes it to London, where the run is officially sold out – but get in if you can. This is an exceptionally intense and powerful piece of theatre.

And what a relief, at last, to have a play about the Iraq war that doesn’t lecture us, with the ghastly smugness of hindsight, on what we all know already: that this war was muddled and ill planned, and that its political leaders were culpably naive, if not downright dishonest. Instead, Burke’s sublimely simple idea was to go to a Fife pub, get some squaddies talking, then turn their accounts into drama. The heart of the story is the Black Watch’s deployment, in 2004, to Camp Dogwood – a dangerous new posting – at the request of the Americans.

Be warned, though: it isn’t as objective as it appears. The soldiers Burke talked to had just left the army, so their views were arguably more jaundiced than those still with the regiment. One private’s opinion that the invasion was more about “bullying than fighting” is taken as the last word, and there is no wider sense that coalition forces are protecting sections of the population against others.

Yet this soldier’s viewpoint is still a blast of fresh air. And not once in two hours do you remember you’re watching actors. You think you’re watching Scottish squaddies, square-bashing, on ops, “on the pish” – the energy and conviction of the ensemble is astonishing. Paul Rattray gives a standout performance as Cammy, embittered but devoted, and Michael Nardone is terrificas the Sergeant. The language is filthy, furious and comically hyperaggressive, and even the fight scenes are convincing. Usually in theatre, it is when the violent squalor of real-life brawling is rendered by some ludicrously balletic mime that the illusion of “real soldiers” breaks down and you are reminded that you are watching actors. Here, a sequence of unarmed combat is a compelling vision of synchronised and patterned aggression.

Black Watch is also acute on male comradeship and esprit de corps. The regiment recruits almost exclusively from the tight-knit communities of Perth, Fife and Dundee, giving it an exceptionally strong sense of identity and camaraderie.At one point, Cammy runs through a potted history of the Black Watch for us, while being dressed, undressed and redressed by his mates in the various period uniforms. His regimental forefathers fought the Americans and the Indians at Yorktown and Ticon-deroga, the French in India and Egypt, and at Waterloo, the Russians at Balaclava and Sevastopol, the Boers in South Africa, the Germans at Mons and Ypres, the Japanese in Burma and the Chinese in Korea, as well as sundry Turks and Arabs along the way. Being a British regiment, they have fought pretty well everyone, though the play doesn’t quite have the attitude to add “and nearly always won”. I suspect any proud Black Watcher would have added this triumphalist note – a further reminder that these are not pure soldiers’ voices delivered straight from source, but filtered through the subsidised theatre.

The sequence when the airmail arrives from wives and girlfriends, and these belligerent, foul-mouthed soldiers fall silent and read, each in his own private world, then drop the letters and send their replies in deaf-and-dumb sign language, is one of the most economical, beautiful and moving I’ve ever seen.

The effect is only magnified by Davey Anderson’s music, superb throughout. He seems to have cobbled together bits of Michael Nyman, Snow Patrol and Scottish ballads with magpie virtuosity, as well as producing haunting arrangements for stomping regimental songs such as The Gallant Forty-Twa, which work brilliantly. His music, like the production generally, is all the more haunting given that, since 2006, the Black Watch has been not a regiment, but only the 3rd Battalion of the newly amalgamated Royal Regiment of Scotland. As well as a powerful portrait, this is an elegy.

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