Well no actually, according to official materials it's an alt history where the Romans were partially driven back by this one dude but for some reason the same equivalent nations and events persisted for the next millenium and a half afterwards, the Tudor Dynasty managed to survive an extra couple centuries, the American Revolution lost because Ben Franklin got bribed with a noble title somehow, the rest of the era of revolution went even further than in real life and managed to simultaneously overthrow all the monarchies in Europe except for Britains and merged togeher into one big democracy blob, and then Napoleon managed to beat Britain so thoroughly that the surviving nobility was forced to flee to the American colonies, gained an unhealthy obsession with Charles Darwin, and rebuilt the empire by taking over the rest of the Americas. Thanks to sakuradite technological development got even more wonky than it looks at first glance and all the guns in the show are actually coilguns. Also for the longest time it was believed that if you convert the calendars over the show actually takes place in the real world's 1960s which was a huge part of why the previous sentence was notable, but apparently that got retconned recently
...It's really best not to try to make sense of the setting of Code Geass.