The Diocese of Pannonia (Latin: Dioecesis Pannoniarum, lit. "Diocese of the Pannonias"), from 379 known as the Diocese of Illyricum, was a diocese of the Late Roman Empire. The seat of the vicarius (governor of the diocese) was Sirmium.
The Diocese of Pannonia was one of the two dioceses in the eastern quarters of the Tetrarchy not belonging to the cultural Greek half of the empire (the other was Dacia), and was permanently transferred to the Western Empire at the death of Theodosius I in 395. Its ultimate fate is uncertain. Pannonia was lost to the Huns in the 440s, although Dalmatia was retained by the Western Empire until ca. 480. The Ostrogoth king of ItalyTheodoric the Great conquered Pannonia in the early 6th century, possibly reestablishing the diocese.
In the 9th century, Diocese of Pannonia was also a name of the ecclesiastical territory of the Christian church whose archbishop was Saint Methodius. [1]
Sources
The Times History of Europe, Times Books, London, 2001.
Provincial administration reformed by Diocletian, ca. 293. Praetorian Prefectures established after the death of Constantine I. Empire permanently partitioned after 395. Exarchates of Ravenna and Africa established after 584. Provinces replaced by Theme system, ca. 680.
* affected (boundaries modified/abolished/renamed) by Justinian's administrative reorganization in 534-536 † re-established after reconquest by the Eastern Empire in 534, as the separate praetorian prefecture of Africa