As a widely-published genre, alternate history is a fairly recent phenomenon. But in a post at io9, David Daw examines the early history of the field, which long predates modern alternate history fiction. He argues that it can be traced back to 1st Century A.D. Roman historian Livy, who speculated about what would have happened had Alexander the Great invaded Roman-dominated Italy. Livy writes:
He would have crossed the sea with his Macedonian veterans, amounting to not more than 30,000 men and 4000 cavalry, mostly Thracian. This formed all his real strength. If he had brought over in addition Persians and Indians and other Orientals, he would have found them a hindrance rather than a help. We must remember also that the Romans had a reserve to draw upon at home, but Alexander, warring on a foreign soil, would have found his army diminished by the wastage of war, as happened afterwards to Hannibal. His men were armed with round shields and long spears, the Romans had the large shield called the scutum, a better protection for the body, and the javelin, a much more effective weapon than the spear whether for hurling or thrusting. In both armies the soldiers fought in line rank by rank, but the Macedonian phalanx lacked mobility and formed a single unit; the Roman army was more elastic, made up of numerous divisions, which could easily act separately or in combination as required.
To read the entire passage in Livy's History of Rome, click on the link, which leads to Book 9 of the work. Then scroll down to sections 9.17 - 9.19.
Link via io9 | Image of Livy via the University of Michigan