Emperor Diocletian’s Tetrarchy (AD 293)
After 284, emperor Diocletian rebuilt the city of Nicomedia (modern Izmit) as his new capital. He divided the Roman empire into East and West regions. He ruled the East, and Maximian ruled the West.
However, by 293 the Roman empire had become too geographically large to be efficiently governed by two men. So Diocletian started a new governing system for the empire called the Tetrarchy, or the “rule by four”—two emperors called Augusti and two co-rulers called caesars.
Emperor Diocletian and caesar Galerius ruled the East from Nicomedia, while emperor Maximian and caesar Constantius ruled the West. Constantius was the father of Constantine the Great.