He joined them with Noricum and Dalmatia to establish the Diocese of Pannonia, with Sirmium as its capital also. In 293, with the establishment of the Tetrarchy, the Roman Empire was split into four parts; Sirmium emerged as one of the four capital cities (along with Trier, Mediolanum, and Nicomedia), and was the capital of emperor Galerius. With the establishment of Praetorian prefectures in 318, the capital of the prefecture of Illyricum was Sirmium, remaining so until 379, when the westernmost Diocese of Illyricum, Pannonia (including Sirmium), was detached and joined to the prefecture of Italia assuming the name of Diocese of Illyricum. The eastern part of Illyricum remained a separate prefecture under the East Roman (Byzantine) Empire with its new capital in Thessalonica. The city also had an imperial palace, a horse-racing arena, a mint, an arena theatre, and a theatre, as well as many workshops, public baths, temples, public palaces and luxury villas. Ancient historian Ammianus Marcellinus called it "the glorious mother of cities". The mint in Sirmium was connected with the mint in Salona and silver mines in the Dinaric Alps through the Via Argentaria. At the end of the 4th century Sirmium came under the sway of the Goths, and later, was again annexed to the East Roman Empire. In 441 the Huns conquered Sirmium; for more than a century it was held by various other tribes, such as the Ostrogoths and Gepids. In 504, Ostrogothic Count Pitzas under Theoderic the Great took Sirmium. For a short time, Sirmium was the centre of the Kingdom of the Gepids and king Cunimund () minted gold coins there. After 567, Sirmium was returned to the East Roman Empire. The Pannonian Avars conquered and destroyed the city in 582. The city was also the location of the Battle of Sirmium that took place in 1167, where a Roman army dispatched by Manuel I Komnenos decisively defeated the forces of Hungary, turning the latter into a satellite state. Roman emperors Ten Roman emperors were born in this city or in its surroundings: Herennius Etruscus (251), Hostilian (251), Decius (249–251), Claudius II (268-270), Quintillus (270), Aurelian (270–275), Probus (276–282), Maximian (285–310), Constantius II (337–361), and Gratian (367–383). The last emperor of the united Roman Empire, Theodosius I (378–395), became emperor in Sirmium. The usurpers Ingenuus and Regalianus also declared themselves emperors in this city (in 260) and many other Roman emperors