Alternate history is fictional history, in which an author changes some aspect of the past and sees how this change would have impacted history as we know it. The Roman historian Livy wrote the first alternate history around 25 BC, when he imagined a world in which Alexander the Great marched West rather than East.
The first mass market alternate history was written in 1836 by a Frenchman named Louis Geoffroy. Call it literary wish fulfillment, the book entitled History of the Universal Monarchy: Napoleon and the Conquest of the World was a smash hit in France.
The first novel-length alternate history written in English appeared in 1895 and was written by an American named Castello Holford. The book called Aristopia (which translated from the Greek means “The best place”) imagines a world in which one of the first English settlers in Virginia discovers a vast reef of gold. The hero uses his new wealth to create a planned society where the state looks after the interests of the vast majority of the people rather than the interests of the very rich. What an imagination!