Why alter history? It’s something you can only do in fiction, and there’s the beginning of an answer in itself.

It is, apart from anything else, a way of celebrating fiction’s power to summon a setting out of thin air, out of mere words, only applied not to a place but to an entire timeline. What was fixed becomes fluid, what was inevitable becomes optional, what could previously be leant on out there in the world – a history everyone shares – becomes the writer’s responsibility to invent, to curate, to edit. Very gratifying to any novelist’s inner megalomaniac.

But what do you get from it, as a storyteller? Well, several different possible things, in line with the wild multiplication of alternative histories and their rise into cultural visibility. There’s a lot of it about. What began in the 1920s and 1930s as an obscure little parlour game for scholars grew up, thanks to a few individually brilliant science fiction novels – Ward Moore’s Bring the Jubilee (1955) and Philip K Dick’s The Man in the High Castle (1962) – into a whole thriving subgenre of written SF, with its own traditions and expectations and (of course) clichés, throwing off from time to time ideas so potent that they burst out in turn into new sub-sub-genres. The whole of steampunk exists because of William Gibson and Bruce Sterling’s alternative history The Difference Engine, published back in the 1980s. And now alt-hist has overflowed genre altogether and become part of the mainstream. What-might-have-been is part of the ordinary array of narrative possibilities in literary fiction, comic books, superhero movies, TV drama. Kate Atkinson’s Life After Life butted up against the door of alt-hist. Sandra Newman’s extraordinary novel The Heavens, which begins in utopia and then decays by stages back into our own history, is alt-hist. Philip Roth wrote some alt-hist (The Plot Against America). Bridgerton is alt-hist, for heavens’ sake.

Some fixed points remain in all this proliferation. The two most common things for alt-hist to change in the past are still, by a large margin, the outcome of the American Civil War and the outcome of World War Two. Victorious Confederates, victorious Nazis, over and over again. Some of this may be a sort of founder effect, a path dependency, a consequence of Moore’s and Dick’s early adventures in alt-hist having happened to tell those two particular stories. But I think it’s the combination in both cases of neat military turning point with vast moral stakes. On the outcome of a battle depends the triumph, or not, of a spectacular and unambiguous evil.

But then beyond the big two come endless other alterations. You can take your pick from unfallen empires (Roman, British) to colonialism run in reverse, from technology slowed to technology accelerated. An abundance of alterations in an abundance of moods, from wry to catastrophic by way of heart-rendingly desirable.

And now the attractions for the writer, in genre and out of it, begin to come into focus. Alt-history writing has its incidental pleasures – a kind of wit you can exercise, in remixing the past, putting familiar things and people in unfamiliar places and vice versa. But at root, and especially when it is taken seriously, it is all about illuminating something in real history. Alt-hist works by resonating with true-hist. It is about showing – sidelong, again – the weight of what really came to pass, by stopping us taking it for granted. By putting the real thing back, defamiliarised, with the apparent inevitability stripped off it, we see it as one possibility among others. We see real history in a space of might-have-beens, and are enabled to make comparisons. This sounds abstract, but it needn’t be; the result can be much richer than a thought experiment, because the relationship to the real past is felt, in good alternative history. It makes a wistful, or a tragic, or an angry, or a yearning acknowledgement of the real course of events.

The best single alt-hist novel I know is Michael Chabon’s The Yiddish Policeman’s Union, set in a Yiddish-speaking enclave on the coast of Alaska which, in this world, has come to be instead of the state of Israel. The book is lovely, wise-cracking noir, with a voice and a protagonist that would be sufficient pleasures in themselves: but the starting point for writing it was Chabon’s discovery of a (real) Yiddish phrase book from the 1950s, which contained useful guidance for calling the police in Yiddish, renewing your passport in Yiddish, crossing a frontier in Yiddish, when all those things presuppose what never existed, a Yiddish nation, a 20th century homeland for European Jews with Yiddish as its mother tongue. So he provided one. It was magic, but painful magic. For the point of a wish that undoes the Holocaust is that a wish can’t undo the Holocaust – except in the 411 pages of a fiction.

Similarly – if it works – I am trying in my own new novel Cahokia Jazz to explore the real weight of real American history, by providing for 475 pages a world where the continent of North America wasn’t conveniently emptied by European diseases as the colonists arrived. In Cahokia Jazz there has had to be a mixture of indigenous and settler cultures, rather than Native Americans being pushed to the margins. I hope that while you read, the city of Cahokia with its ancient core and its factories feels real. Real enough, so that when my riff on the old standard tune of American history is over, and you look up from the busy streets with the clanging streetcar bells, and find that in its place there is only a quiet archaeological site beside the Mississippi river – the absence of all that life suddenly seems strange.