The Prince of Homburg, Donmar Warehouse, London

Heinrich von Kleist's The Prince of Homburg (1811) is one of those deeply ambiguous works that can be co-opted by diametrically opposed political factions. It's said to have been a favourite of Hitler's and insofar as it dramatises the triumph of Prussian military discipline over individual impulse, you can see why the Nazis were keen to appropriate it. But it is also a lethal indictment of this nascent ideology. It may sound a queer objection to complain that Hitler could never have been hoodwinked into approving of this new version by Dennis Kelly. But in equipping the play with a radically different conclusion, this distorting adaptation robs the Kleist original of its subtlety, symmetry, and enigmatic equipoise.

Director Jonathan Munby's last show at this address was Calderón's Life Is a Dream, with which The Prince of Homburg has strong affinities in the shape of a protagonist who is likewise reduced to a politically motivated confusion between the dream state and reality. In Kleist's opening scene, the Prince (Charlie Cox) is discovered sleepwalking in a moonlit garden and weaving a laurel wreath as he imagines glory in the following day's battle against the Swedes.

What he does not realise is that the Elector and his beautiful niece intervene in these somnambulistic fantasies in order to test the young man's ambition. The Prince emerges from this experience too distracted to attend to military instructions. Ironically, he wins a heroic victory through disobeying orders and finds himself court-martialled and sentenced to death by the Elector. Pestered by petitions for clemency, the ruler then adopts a cagey strategy of offering to rescind his verdict if the Prince can prove that it is unjust.

Charlie Cox strikingly captures the hero's existential perplexity as he progresses from dashing romantic innocence through desperate, death-haunted compromise to born-again Prussian conformity. Eerie song and balefully subjective lighting atmospherically establish the treacherous nature of the "reality" the protagonist is forced to traverse. But Kelly's adaptation does not trust the fact that Kleist's condemnation of the military ethos is all the more devastating for being so studiedly equivocal. I have no quarrel with the way that, though the production is set in the period of the play's composition, Prussia is referred to as "the fatherland" or that the heel-clicking soldiers have a proto-Nazi verbal salute.

Where the version traduces the original is in substituting a tragic ending that absolves the drama from the taint of its future associations. Kleist's close, in contrast, unnerves by its ostensible, state-sanctioned happiness: it brilliantly mirrors the somnambulistic opening and leaves one wondering whether the Prince's surrender to obedience has been engineered by playing an equivalently bamboozling and shoddy trick on his susceptibilities. To expedite Kelly's changes, Ian McDiarmid's compelling stickler of an Elector has to coarsen from sardonically self-aware manipulator with mysteriously mixed motives to hectoring fascist devoid of paternal interest in the Prince. Given that this is a rare English revival of a German classic, would it not have been a courtesy to the audience to indicate in the programme that not all of it is echt Kleist?

To 4 September (0844 871 7624)