Sirmium), was dettached and joined to the Praetorian prefecture of Italia assuming the name of Diocese of Illyricum. The eastern part of Illyricum remained a separate prefecture under the East Roman Empire with its new capital in Thessalonica.
From the 4th century, the city was an important Christian center, and the seat of the Bishop of Sirmium. Five church councils, the Councils of Sirmium, were held in Sirmium. The city also had an emperor's palace, horse racing arena, mint, arena theatre, 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".
At the end of the 4th century, Sirmium was brought under the sway of the Goths, and later, was again annexed to the Eastern Roman Empire. In 441, Sirmium was conquered by the Huns, and after this conquest, it remained for more than a century in the hands of various other tribes, such as Eastern Goths and Gepids. For a short time, Sirmium was the centre of the Gepid State and King Cunimund minted golden coins there. After 567, Sirmium reverted to the Eastern Roman Empire. The city was finally conquered and destroyed by the Avars in 582.
(AVAR RULE) (Eurasian Avars) (Avar Khaganate) (Siege of Sirmium)
(panacomp.net/Serbia - Sirmijum) (Edward Gibbon, The Decline and Fall of the Roman Empire)
Baynes, Norman H. (1913), "Chapter IX. The Successors of Justinian", The Cambridge Medieval History, Vol. II: The rise of the Saracens and the foundation of the Western Empire, New York: Cambridge University