The Germans have won the Second World War. Parliament is festooned with Swastikas. What would you do?

With the dramatisation of SS-GB starting tonight, David Barnett wonders ... would you be the partisan hiding in the hills, harrying the occupying forces, would you rather die than live under the jackboot of fascism, or would you collaborate?

Buckingham Palace is bombed out and German storm-troopers are patrolling the streets of London (BBC)

When Len Deighton’s novel SS-GB was first published almost 40 years ago, its central conceit was almost unthinkable: what if the Nazis had won the Second World War, and occupied Britain? A BBC adaptation of the novel begins tonight, and the stark imagery it portrays includes a crumbling, bombed-out Buckingham Palace, German storm-troopers patrolling the streets of London, and the Houses of Parliament festooned with swastikas.

SS-GB follows Douglas Archer (Sam Riley), a Scotland Yard police detective who, following the fall of London in 1941, finds himself effectively working for the Gestapo. He’s a good man in a bad situation; compelled to serve justice, but wondering who justice now serves.

SS-GB is one of the most well-known entries in a literary sub-genre often called counterfactuals or alternate histories… stories which take a point where history diverges from what we know happened to what could have happened; in the case of SS-GB and many other novels, the Nazis winning the war. Another fine example is the science fiction writer Philip K Dick’s The Man in the High Castle, which is set in an America divided up by triumphant Nazis and Japanese forces, which has also made the leap to TV. It’s first series was Amazon Prime’s most popular show ever.

Robert Harris’s Fatherland, published in 1992 and made into a successful TV movie by HBO in 1994, told the story of a German win from within the Third Reich, set in 1964 amid the preparations for celebrating the (still surviving) Adolf Hitler’s 75th birthday, and one criminal investigator uncovering what, in the light of a Nazi triumph, was still suppressed from most of Germany and the rest of the world – the Holocaust.

Sam Riley plays Douglas Archer, a Scotland Yard police detective who works for the Gestapo (BBC)

In film, too, the idea of a Nazi victory has appalled and fascinated. Perhaps the earliest example is a movie that was produced while the war was still being fought, in 1942; Went The Day Well, adapted from a Graham Greene story by Ealing Studios, in which a force of German paratroopers, disguised as British soldiers, invade the fictional village of Bramley End.

The plot is routed, of course, and the movie was a rousing bit of propaganda. Not so 1964’s It Happened Here. Riffing off a 1936 novel It Can’t Happen Here by Sinclair Lewis — which had on its first edition the cover line “What will happen when America has a dictator?” — the movie is a bleak look at life in England under Nazi rule in 1944, following a successful invasion of Britain in 1940 after a disastrous retreat from Dunkirk.

It Happened Here goes for less of a slick thriller vibe than perhaps SS-GB… the government is nominally the British Union of Fascists, the streets are patrolled by British Blackshirts, and Oswald Moseley’s portrait hangs alongside Hitler’s.

But what both works do is ask a difficult question; in such a situation, what would you do? Perhaps we all like to think that we’d be the partisans hiding in the hills, harrying the occupying forces. That we’d be living double lives as quiet citizens and agents of the resistance. That we’d rather die than live under the jackboot of fascism. Or would we be the collaborators who work with the new world order, because we have mouths to feed and we don’t want to be lined up against a wall and shot? And because the country needs to keep running, whoever’s in charge?

Perhaps those images of swastikas rippling over well-known landmarks in the free world have lost some of their potency since the time when Len Deighton wrote SS-GB. Perhaps we’re not as shocked, as chilled to the core, as we once were. Indeed, we’ve had a rise in instances of swastikas being daubed in public places in the UK post-Brexit referendum, and in the US, after Donald Trump’s Presidential triumph.

This week, the symbol was painted on the campus of Exeter University. One was sprayed on the side of a car in a predominantly Jewish neighbourhood in Florida. Commuters in New York awoke to swastikas and anti-Semitic graffiti adorning subway trains this month. The Nazi emblem was painted, along with the words “Go Trump!” in a children’s park in Brooklyn named after dead Beastie Boys musician Adam Yauch back in November. The list goes on.

Sam Riley and Kate Bosworth in SS-GB (BBC)

Perhaps those perpetrating these outrages are just too young to appreciate the full, potent evil of the symbol they employ with such abandon. Or maybe they’re fully aware, and deploy it to maximum effect because that’s the world we’re creating right now. There have been responses of hope. Those commuters faced with the graffiti on their morning trains banded together, unasked by anyone, to clean up the carriages. A rally was held in Adam Yauch Park in the aftermath of the attack.

But comparisons between the situation in the US and the UK today and Germany in the 1930s — that led to the rise of Hitler — abound. Attacks, both political and actual, on minorities and women and the free press are comparisons a lot of people are drawing. But, we still tell ourselves, it can’t happen here, right?

A couple of weeks ago the American TV writer David Slack posted something on Twitter which was, quite rightly, shared many tens of thousands of times. He wrote: “Remember sitting in history, thinking ‘If I was alive then, I would’ve…’ You’re alive now. Whatever you’re doing is what you would’ve done.”

We are often told that we have to learn the lessons of history to avoid repeating the mistakes. Perhaps it’s not just a warning from history we have to heed; the fluttering swastikas over London landmarks in SS-GB and its ilk might be a sign we also have to consider the history that never happened to avoid being complacent about the future we might just get.

SS-GB is on BBC1 at 9pm