War should be no laughing matter and yet satirists continue to deliver the goods, in part, because America simply can't live with peace; the title of Australian writer-director David Michôd's comedy neatly sums up the superpower's boneheaded and highly lucrative obsession with invasion. Michael Hastings' non-fiction book The Operators provides the inspiration and star Brad Pitt throws himself full-pelt into his performance – a performance that, unfortunately, constitutes a massive misjudgement.

Pitt plays four-star general Glen McMahon (a character based on General Stanley McChrystal), who is brought in to lead US forces in Afghanistan in 2009. Evidently honourable, he's nevertheless deluded enough to think he can fix the unfixable. The loyal team that flank him include body man Willy (Emory Cohen) and Topher Grace's PR consultant. McMahon is desperate to secure additional troops and makes earnest efforts to consult the Afghan president (Ben Kingsley) – who considers this a token measure – while a visit from wife Jeannie (Meg Tilly) emphasises the personal cost of McMahon's devotion to his country.

Michôd mounts a sometimes sophisticated critique of US warmongering. Scenes where exhausted troops attempt to extract clarity regarding their mission, and where they blunder into a village, highlight the folly of their continued presence – the risibility of invaders trying to win hearts and minds. Britain's Will Poulter and Get Out's Lakeith Stanfield bring palpable humanity to the men on the frontline and the film is slickly shot by Ridley Scott regular Dariusz Wolski, with narration from Scoot McNairy that's by turns wry and wistful.

What a shame then that Pitt – an actor of unquestionable charisma for whom less is invariably more – plumps for a SNL-style caricature that's part Popeye, part his Aldo Raine from Inglourious Basterds, and that features a lot of squinting and surveying. Presumably aiming for something Dr Strangelove-esque and going horribly awry, the perfectly preserved A-lister visibly strains to appear gruff and wizened, succeeding only in looking bonkers. It's not a performance without purpose, however, its very anomalousness brands McMahon a relic and illustrates the huge remove from reality at which the US forces operate.

Michôd previously gave us the masterful crime thriller Animal Kingdom and well-executed dystopian drama The Rover. He's such an assured director that, despite his leading man's awkward efforts, War Machine never feels like a mess, more an oddity; it might not say much that is new but it often says it in a way that hits home. Pitt does dial it down eventually, adding a note or two of pathos, but the spectacle of his effortful work is a major distraction that leaves most of the supporting cast looking on in bewilderment.

Screening in selected cinemas and on Netflix from Fri 26 May.