What if the Gauls had defeated and killed Julius Caesar at the Battle of Alesia in September, 52 BC? This may seem far-fetched given what comes to mind when one imagines Roman military prowess, but the Gauls outnumbered the Romans as much as four to one at times and events were very much in doubt throughout the battle; Caesar himself having a close call with death.

This would have stopped the early expansion of Rome into Gaul (modern day France) rewriting the entirety of western history. Of course, it’s possible Pompey would have taken the helm and Rome would have eventually returned to fight Gaul.
What we do know: Caesar’s victory in Alesia was the end of Gaulish liberty and laid the foundation for Empire and the destruction of the Roman Republic.

“Battles are singular moments in history, productive of strange events. Much may depend upon a small detail, the effects of a detail may be victory, and the effects of victory may be long lasting. Alesia was such, for in a very real sense, it symbolized the extinction of Gaulish liberty. Rebellions would come and go, but never again would a Gaulish warlord, independent of Rome, hold sway over the tribes of Gaul. To gain liberty, Vercingetorix, a strong and popular leader, had hazarded everything at Alesia, and lost.”

Alesia 52 BC: The final struggle for Gaul (p82)