Battle of the Catalaunian Plains

The Battle of the Catalaunian Plains was a major battle in the late stages of the fall of the Western Roman Empire, fought in what is now France between Roman- and Hun-led alliances on 20 June 451 CE.

In the alternate timeline that would later spawn the mirror universe, Roman General Flavius Aetius overthrew his liege Emperor Valentinian III following the battle, which led to the Western Empire surviving a little longer than in the prime universe.