Diocletian divided the Roman Empire into four rulers through the Tetrarchy in 293 CE.
The system placed two senior emperors, called Augusti, above two junior emperors, called Caesars. Diocletian ruled the eastern empire, while Maximian governed the west. Galerius and Constantius Chlorus served as their Caesars. The arrangement was intended to improve administration and make frontier defense faster.
The Tetrarchy did not permanently divide the empire into four independent states. It was designed as a cooperative imperial college, although rivalries soon weakened it. After Diocletian and Maximian abdicated in 305, political conflict eventually allowed Constantine the Great to become sole emperor.
Diocletian’s reforms also included tax, military, and administrative changes. He is often associated with the empire’s recovery from the third-century crisis, despite his persecution of Christians and his unusually elaborate imperial court.