What is the oldest AH book?

Does anyone know what book might be the oldest in the alternate history genre, or at least what is the oldest book to feature alternate history elements?

I would suggest maybe Aristopia by Castello Holford is a good contender. It was published in 1895 and was more utopian fiction than alternate history, but I suppose the genres can sometimes go hand in hand.
I believe that Plato or some other Ancient Greek or Roman speculated on what would've happened if the Persians won at Platea or if Alexander went west.
But Utopia is basically a made up place to serve in a parable instead of some story based around 'What if this event here went different?'. Also even if it counts, Plato's Atlantis would be a much earlier example.
yes that story crossed my mind too
If Utopia's alternate history, then every myth and legend that mentions the past or a fictional place is also alternate history.
There's a 19th century French book titled Uchronie dealing with a world where Commodus not succeeding Marcus Aurelius, leading to a lasting Roman Empire and faster overall progress. I don't remember the author's name but I read it mentioned as considered the earliest example of an actual AH, at least in Western literatures (not counting Livy's discussion of Alexander going west, or other hypotheticals in the context of historical works).
However, I also recall reading about an earlier work, also in French, about a Napoleonic victory, written during the Restoration (and followed by a few similar others). So, AH literary fiction seems to emerge in France as a way to reflect about the Revolutionary period after is end.
It may be that other, more obscure earlier examples exist though.
Most people credit the concept of a time machine to HG Wells, while a Spanish playwright named Enrique Gaspar had introduced the idea in a play a few years prior.
Surprised no-one's yet linked Uchronia -- that page agrees that the first work to feature alternate history is Ab Urbe Condita Libri, while the first novel-length alternate history work is Histoire de la Monarchie universelle: Napoléon et la conquête du monde by Louis Geoffroy (written in 1836). The latter seems to be the work Falecius was thinking of.
Swastika Night could count as an honorary example from 1937, featuring a world where the Nazis and Imperial Japanese won a then-future Second World War. Europe and Africa are ruled by a neo-feudal German Empire where most of the population are illiterate peasants lorded over by S.S. knights. Hitler is remembered as a tall, blonde Aryan superman who personally won World War 2 with a bombing run over Moscow, among other things.