Ruler who divided the roman empire in half?

Emperor Diocletian in CE 285. However, the empire was reunified for short periods of time, like the reign of Emperors Constantine, Julian the Apostate, and Theodosius I. Also, the Eastern Empire had re-invaded the Western Empire during the reign of Justinian I and tried to recreate and rebuild the empire, but with his death the Western part of the Byzantine Empire was retaken by Barbarians.