Eastern Illyricum

Praetorian prefecture of Illyricum Praefectura praetorio per Illyricum Ἐπαρχότης Ἰλλυρικοῦ
Praet. prefecture of the East Roman Empire
347–7th century
The praetorian prefecture of Illyricum (375-379)
Capital		Sirmium, later Thessalonica
Historical era		Late Antiquity
•	Established	347
•	loss of most of Illyricum to Slavic incursions	7th century
Political subdivisions		Diocese of Macedonia Diocese of Dacia Diocese of Pannonia (until 379)

The praetorian prefecture of Illyricum (Latin: praefectura praetorio per Illyricum, Greek: ἐπαρχότης/ὑπαρχία [τῶν πραιτωρίων] τοῦ Ἰλλυρικοῦ, also termed simply the Prefecture of Illyricum) was one of four praetorian prefectures into which the Late Roman Empire was divided.

The administrative centre of the prefecture was Sirmium (375-379), and, after 379, Thessalonica. It took its name from the older province of Illyricum, which in turn was named after ancient Illyria, and in its greatest expanse encompassed Pannonia, Noricum, Crete, and most of the Balkan peninsula except for Thrace.

Unlike the other three "classical" prefectures that are mentioned in the Notitia Dignitatum, the (Gaul, the Italy-Africa and the East), the early administrative history of Illyricum as a prefecture during the 4th century involved its abolition, re-establishment and division several times.

Initially the territories comprising the later praetorian prefecture of Illyricum belonged to the Prefecture of Italy, Illyricum and Africa. It was as established as a praetorian prefecture in its own right during the dynastic struggles between the sons of Constantine the Great which followed his death in 337. It seems that the three dioceses of Macedonia, Dacia and Pannonia were first grouped together in a separate praetorian prefecture in 347 by Constans by removing them from the praetorian prefecture of Italy, Africa and Illyricum (which then became the praetorian prefecture of Italy and Africa) or that this praetorian prefecture was formed in 343 when Constans appointed a prefect for Italy.

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