Western Roman Empire capital Mediolanum
Tetrarchy
Diocletian was the first emperor to divide the Roman Empire into a Tetrarchy. In 286 he elevated Maximian to the rank of Augustus (emperor) and gave him control of the Western Empire while he himself ruled the East. In 293, Galerius and Constantius Chlorus were appointed as their subordinates (caesars), creating the First Tetrarchy. This system effectively divided the Empire into four major regions, as a way to avoid the civil unrest that had marked the 3rd century. In the West, Maximian made Mediolanum (now Milan) his capital, and Constantius made Trier his. In the East, Galerius made his capital Sirmium and Diocletian made Nicomedia his. On 1 May 305, Diocletian and Maximian abdicated, replaced by Galerius and Constantius, who appointed Maximinus II and Valerius Severus, respectively, as their caesars, creating the Second Tetrarchy.The Tetrarchy collapsed after the unexpected death of Constantius in 306. His son, Constantine the Great, was declared Western emperor by the British legions, but several other claimants arose and attempted to seize the Western Empire. In 308, Galerius held a meeting at Carnuntum, where he revived the Tetrarchy by dividing the Western Empire between Constantine and Licinius. However, Constantine was more interested in conquering the whole empire than he was in the stability of the Tetrarchy, and by 314, began to compete against Licinius. Constantine defeated Licinius in 324, at the Battle of Chrysopolis, where Licinius was taken prisoner, and later murdered. After Constantine unified the empire, he refounded the city of Byzantium in modern-day Turkey as Nova Roma ("New Rome"), later called Constantinople, and made it the capital of the Roman Empire. The Tetrarchy was ended, although the concept of physically splitting the Roman Empire between two emperors remained. Although several powerful emperors unified both parts of the empire, this generally reverted into an empire divided into an East and a West upon their deaths, as happened after the deaths of Constantine and Theodosius I.
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