It's topical to be thinking about that strange genre known as 'What If?', or Alternative History, when the new BBC TV adaptation of Len Deighton's SS-GB is about to appear. This goes back a long way, I think I read it in the 1980s. Thankfully, I can't remember anything about the plot, since I'm looking forward to seeing it.
A few other topics crop up time and time again. Partly because of my own interests, I've noticed a great deal of what-if? devoted to the US Civil War, and to the possibility of the Southern States winning it. The fact is, inconvenient to many deluded 'romantics', that the Confederacy only ever had a slim-to-non-existent chance of surviving that war. I can, sadly, see what the fascination is: the war brought into sharp definition the South as a distinct society, which if you ignore what is very much the elephant in the room, namely the appalling institution of slavery, has a romantic air flavoured with the chivalry of a mythical past, and yet is extinguished for good only a few years later. If you don't know what I'm alluding to, just watch Gone With The Wind. One example of this sub-genre I've read is The Guns of the South by Harry Turtledove. He is, by the way, one of the kings of what-if?, having written a variety of alternative histories, especially about the First and Second World Wars.
What-if? is in my view not just a branch of historical fiction, but actually a cross-over with science fiction. Alternative histories are typically rationalised in some way, by means of some almost Hitchcockian 'macguffin' to explain the different course events take. One Turtledove book, again about the Civil War, depends on a vital message to Robert E. Lee not being intercepted as in reality it was. But in The Guns of the South, it turns out to be a band of far-right Afrikaaner racists which travels back in time to provide the Confederacy with advanced weapons, and so win the war. There are some twists which please modern sensibilities; but with all of these books you can never quite escape a host of dubious moral conceits. Turtledove is very readable, but he is to be read with caution.
Lastly, let's direct our attention to a classic work of fiction, The Man in the High Castle by Philip K. Dick, which is also a current broadcast drama, and highly praised, which is aggravating to me because I'd love to see it but I don't receive the relevant tv channel. Technically, this is a what-if?, another one featuring a world in which the Axis won the Second World War. And yet, it's something rather different, something which amongst other things examines perceptions of these alternative histories. Philip K. Dick was one of the truly great science fiction writers, a great writer full stop, and this may have been his best known book. From the SF point of view, alternative histories are really takes on themes of multiple or parallel universes. To sign off this piece, I'd just like to say that from my reader's point of view, I wish authors would give WW2 and the US Civil War a rest, and look at some other sudden swerves in history. How about a fiction in which the 2016 US election was not won by a serious politician but an insane demagogue... Whoops.