Fascism in fiction has been in vogue for a while now: the television versions of The Handmaid’s Tale and The Man in the High Castle, Penguin’s republication (on the day of Trump’s inauguration) of Sinclair Lewis’s It Can’t Happen Here, people scurrying to the bookshelves to note all the pre-echoes of Steve Bannon’s politics in Philip Roth’s The Plot against America. I don’t know what emotional need these might-have-beens and could-it-yet-bes serve, unless it’s a version of ferreting around in Nostradamus for strings of words that might be contorted into a prediction of something that’s just happened: things feel more manageable when you can tell yourself that someone saw this coming.

Kevin Brownlow and Andrew Mollo’s 1965 film It Happened Here, just released on disc by the BFI, is one of the founding documents of the ‘What if the Nazis had won?’ sub-genre, always popular in the UK: it was followed by Len Deighton’s novel SS-GB (1978, adapted for television in 2017), Philip Mackie’s BBC TV serial An Englishman’s Castle (also 1978, with a DVD release in 2016) and Robert Harris’s Fatherland (1992). But none of those is as startling as the newsreel realism of It Happened Here, with its scenes of Nazi soldiers marching through Whitehall, riding double-decker buses, flirting with English girls and patrolling the streets alongside London bobbies.

Brownlow was still at school when he came up with the idea of a film about Nazi Britain, and began shooting at weekends; the story came later, as did Mollo, who brought an eye for detail and an absolute distaste for anachronism. The flawless sense of period is one source of the film’s power, not just in the 1940s clothes and hairstyles but the tone of voice on radio broadcasts, the sense of perpetual insufficiency: nobody looks implausibly well fed or well dressed. Towards the end of filming, as Brownlow and Mollo progressed through jobs in the lower reaches of the British film industry, they came across people of importance who offered help: Kubrick donated all the unexposed parts of film reels from Dr Strangelove, and Tony Richardson – fresh from the triumph of Tom Jones – used his clout to get them a distributor. But for the most part they relied on the good will of friends and acquaintances as cast and crew; the only professional actor was Sebastian Shaw, playing a doctor secretly working for the resistance (he later gained more lasting fame as the unmasked face of Darth Vader in Return of the Jedi – more stormtroopers). The budget for the whole 97 minutes, stretched across eight years, came in at around £7000; only the sound, mostly dubbed on in post-production, gives away the cheapness.

The action is set in 1944: the Nazi regime that has been running Britain for four years is struggling with partisans and the threat of American invasion. Pauline, a nurse (Pauline Murray), having witnessed the slaughter of friends by partisans, determines to get on with her life, shunning politics. To carry on working, she has to join Immediate Action, a Nazi organisation. The scenes of her induction are slyly reminiscent of such wartime propaganda films as Carol Reed’s The Way Ahead (1944), in which an ill-assorted bunch of troops are welded into a smart fighting unit (that film’s closing sequence, in which the platoon marches forward through smoke and dust, was imitated for Dad’s Army). The most obvious difference is the way training is punctuated with Nazi indoctrination classes and cultish rituals involving torchlight, darkness and allusions to sacrifice and the fatherland. A less acute fiction would have shown Pauline plucking up her courage and taking sides; but she muddles on, ever more tainted.

Over lunch in the Immediate Action canteen, Pauline and another woman question a group of dedicated Nazis about their beliefs. They talk enthusiastically about the need to dispose of people who are a burden on the state (‘a surgical operation, getting rid of useless matter, useless tissue’), and the nature of ‘the Jew’: ‘Every race is superior to the Jew. The Jew has no home. The Jew is a parasite race. The Jew waits for a civilisation to be established and then establishes himself on it, a flea on a dog.’ The horror lies in the unforced conviction with which the men express these views. The actors were actual Nazis, part of a group that had approached Brownlow to borrow SS uniforms for a party (it turned out to be a 20 April party, in honour of Hitler’s birthday). Brownlow felt that what they said was self-evidently repellent, and that to have another character argue with them would be out of keeping – in a totalitarian state, who argues? Others, including Jewish groups, were horrified that the Nazis were not contradicted, and United Artists insisted on cutting the scene for general release.

It confirms the movie’s integrity that you can read such contrary lessons into it: it’s a film about how easily people find they can tolerate the intolerable, and about the deep roots of anti-Semitism in British society; or it’s a demonstration of the need to talk without constraint about anti-Semitism; it’s a film about the evils of violence or the necessity of it. However you take it, who now is going to maintain with any confidence that it can’t happen here?