293, Diocletian thought that more focus was needed on both civic and military problems, so with Maximian's consent, he expanded the imperial college by appointing two caesares (one responsible to each augustus)—Galerius and Constantius I. In 305, the senior emperors jointly abdicated and retired, allowing Constantius and Galerius to be elevated in rank to augustus. They in turn appointed two new caesares—Severus II in the west under Constantius, and Maximinus in the east under Galerius—thereby creating the second Tetrarchy. Regions and capitals The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notably Sassanian Persia) and barbarians (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at the Rhine and Danube. These centres are known as the tetrarchic capitals. Although Rome ceased to be an operational capital, Rome continued to be nominal capital of the entire Roman Empire, not reduced to the status of a province but under its own, unique Prefect of the city (praefectus urbi, later copied in Constantinople). The four tetrarchic capitals were: Nicomedia in northwestern Asia Minor (modern İzmit in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids was the capital of Diocletian, the eastern (and most senior) augustus; in the final reorganisation by Constantine the Great, in 318, the equivalent of his domain, facing the most redoubtable foreign enemy, Sassanid Persia, became the praetorian prefecture Oriens, 'the East', the core of later Byzantium. Sirmium (modern Sremska Mitrovica in the Vojvodina region of modern Serbia, and near Belgrade, on the Danube border) was the capital of Galerius, the eastern caesar; this was to become the Balkans-Danube prefecture Illyricum. Mediolanum (modern Milan, near the Alps) was the capital of Maximian, the western augustus; his domain became "Italia et Africa", with only a short exterior border. Augusta Treverorum (modern Trier, in Germany) was the capital of Constantius, the western caesar, near the strategic Rhine border; it had been the capital of Gallic emperor Tetricus I. This quarter became the prefecture Galliae. Aquileia, a port on the Adriatic coast, and Eboracum (modern York, in northern England near the Celtic tribes of modern Scotland and Ireland), were also significant centres for Maximian and Constantius respectively. In terms of regional jurisdiction there was no precise division among the four tetrarchs, and