Unfortunately for our understanding of Britain’s past and our estimate of its likely future, the Oscars have no category for the untrue: “for the movie”, as the host might put it as he or she fingers the envelope, “that most energetically disrespects the historical record”. If there were, two contenders high on this year’s fibbers’ list would be Dunkirk and Darkest Hour – films that between them have gathered 14 Oscar nominations.

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Dunkirk has eight, including those for best picture and best director (Christopher Nolan); Darkest Hour has six, including best picture and best actor (Gary Oldman). Together they burnish the story of British resolve in the summer of 1940: first in evacuating the army from the beaches of France; second in carrying on the struggle against Nazi Germany with no ally outside the empire.

Both films were conceived some time before the EU referendum in June 2016. Nolan finished his screenplay for Dunkirk in 2015 and began shooting in May 2016. The producers of Darkest Hour acquired a screenplay by Anthony McCarten in 2015; Oldman began talks about playing Churchill a year later. Another, less successful film, simply titled Churchill (played this time by Brian Cox), got going slightly earlier.

Was there something in the air? Boris Johnson’s breezy Churchill biography appeared late in 2014 and sold surprisingly well, given that it said nothing new and that the production of literary Churchilliana has been a constant in British publishing for 70-odd years. Dunkirk seems to have been an obsession of its director since the 1990s. The origins of these films, as far as one can tell, had nothing to do with Europhobia or the nationalism that was gathering in provincial England, far away from the kind of people who could finance and direct them.

Nonetheless, the films have come to be seen as a reflection and endorsement of the Brexit mood, with headlines in the pro-Brexit press such as “For Brexit to work we need the Dunkirk spirit, not Naysaying Nellies” and “‘We will channel Churchill’ – Brexiteers to warn Michel Barnier”. According to reports, Churchill’s “fight them on the beaches” speech that closes Darkest Hour has brought cinema audiences to their feet; and in Dunkirk there has been applause when an officer, asked why he will stick it out on the bomb-strafed pier, replies with the word “Hope”.

These reactions suggest an England congratulating itself on its past – an idealised past, shorn of inconvenient fact. The films, especially Dunkirk, are sometimes brilliant cinematically, and yet utterly conventional in their patriotism and presentation of character. A film such as The Cruel Sea, made 65 years ago, gave my generation of children just as shocking an idea of what war meant, but refrained from a patriotic Elgar pastiche on the soundtrack (to which Dunkirk unfolds). Still, Dunkirk’s untruths are mainly distortion rather than invention. The British army’s escape from the beaches, for example, was enabled much more by the Royal Navy and an evacuation fleet that included 39 destroyers than by the gallant “little ships” from the rivers and harbours of southern England; and enabled most of all, of course, by a German army that didn’t press on to the coast.

Facebook Twitter Pinterest Winston Churchill’s funeral cortege in London, in 1965. Photograph: AFP/Getty Images

The case against Darkest Hour is more serious. Did Winston Churchill in the summer of 1940, or any other time, ever take a tube? Did he on that tube talk in a friendly way to his fellow passengers, whose mouths hung open when they saw him? Did he ask for their names and opinions on whether Britain should fight, or sue for peace? Did he quote aloud from Thomas Macaulay’s Lays of Ancient Rome, to have the verse completed by a young black man, whose hand Churchill amicably touched? And then did he return to Westminster to announce his renewed resolve in a speech to parliamentarians that quoted the names and opinions of the Londoners he had met on the train, very much as Jeremy Corbyn might have done (“Jenny Brown from Walthamstow writes to say …”)?

No, he did none of these things. The scene was absurd. Half of me minded: the half that conflated this fiction with Brexit and agreed with the historian Max Hastings, writing in the New York Review of Books, that while Dunkirk might be a decent educational tool for children, it should have been forcibly denied to grown-ups “at this moment when we are threatened with embarkation upon one of the most self-indulgent, wilfully foolish acts of self-harm in the nation’s history”.

But the other half didn’t mind. Both films tend to the theatrical. The Darkest Hour is dark indeed – the Commons chamber looks as though candles light it – while tracking shots capture London street scenes that might have been directed by Franco Zeffirelli. In one of them, the camera follows a dustman moving towards a dustcart with a full dustbin, out of which a 1940s cabbage leaf nicely protrudes. It must be glued on, you think, as you watch and wonder how many takes it has taken to get right: “Cut! Can’t see the cabbage leaf …”, and so on.

Once Brexit is taken out of the picture, these films look more like a national obituary than a call to arms – to remind the audience, to quote William Manchester on Churchill’s funeral, that they “mourned, not only him and all he meant, but all that they had been, and no longer were, and would never be again”.

Once Brexit is taken out of the picture, these films look more like a national obituary than a call to arms

Two hundred years ago the world’s first great historical novelist, Walter Scott, did something similar for a nation by writing books about a tribal, pre-industrial Scotland that had largely disappeared. The present-day Scottish writer Tom Nairn called him a “valedictory realist” – a writer who never intended his “unmatched evocation of a national past” to serve “political or social mobilisation in the present”, but rather to stress that, however splendid, it was beyond recall. Scott, after all, was an ardent unionist and lived in a country that, partly thanks to the union, was beginning to see a prosperous future of factory smoke and steam.

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In Scotland, the nationalist impulse was confined for the next century to an antiquarian, often kitschy interest in the folksy history that Scott described. In Europe, his effect was different. There, as Nairn writes, the whole point of cultural nationalism was the resuscitation of the mythical past, to serve present and future political needs, and it was there, and not in Scotland, that Scott made his greatest contribution to the rise of national feeling, with poetry and novels (often translated into operas) that somehow spoke to the local causes.

This is a long way from the Dunkirk beaches and Churchill’s cigar, but it suggests that when it comes to the political effects of art, context is everything. Had remain won, we would be watching a harmless elegy: all that we had been and would never be again.

• Ian Jack is a Guardian columnist