Praetorian prefecture of Illyricum
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 (318-379), 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.
Administrative history 
Unlike the other three "classical" prefectures that are mentioned in the Notitia Dignitatum (Gaul, Italy-Africa and the East), the early administrative history of Illyricum as a prefecture during the 4th century was a matter of it being abolished, re-established and divided several times.
Initially the territories comprising the later prefecture belonged to the central Prefecture of Italy, Illyricum and Africa, when it was established in 337 following the division of the Empire among the sons of Constantine the Great. It seems that the three dioceses of Macedonia, Dacia and Pannonia were first grouped together in a separate praetorian prefecture in 347 by Constans from the prefecture of Italy and Africa.
It remained in existence until 361, when it was abolished by emperor Julian, and then existed again from 375-379 under Gratian. In that year the Diocese of Pannonia (Illyricum occidentale, "Western Illyricum") was again added to Italy as the "Diocese of Illyricum", while Macedonia and Dacia (Illyricum orientale, "Eastern Illyricum") were ruled briefly directly by Theodosius I from Thessalonica. During the years (384-395) they too were incorporated in the Italian prefecture, except a short period in 388-391, when the two dioceses formed a separate prefecture.
Only after the death of Theodosius in 395 and the permanent division of the Empire did the Illyricum assume the permanent form which appears in the Notitia, incorporating the dioceses of Macedonia and Dacia, with Thessalonica as capital. However, the Western Empire, especially during the regency of Stilicho, continued to lay claim to them until 437, when, as part of the dowry of Licinia Eudoxia, Valentinian III recognized the East's sovereignty over the prefecture. On this occasion, it appears that the prefecture's capital was again moved for a while (437-441) to Sirmium, but the move is debated, as the northern Balkans were at the time ravaged by invasions. Likewise, the intention of Justinian I to move the capital to his new city of Justiniana Prima in the 540s remained unfulfilled. 
Following the Slavic invasions in the 7th century, most of the Balkan hinterland was lost by the Byzantines, who only retained control of the parts of Thrace nearest Constantinople, Thessalonica and its environs, and some coastal strips in Greece. A praetorian prefect (ὕπαρχος) is attested in the sources as governor of Thessalonica as late as the first years of the 9th century, one of the last survivals of the old Constantinian administrative system in the entire Empire. At that point however, the wars with the rising power of Bulgaria necessitated a reorganization of the provinces, and Thessalonica was constituted as a distinct theme under a strategos sometime before 840.
List of known praefecti praetorio per Illyricum 
- Vulcacius Rufinus (347-352)
- Anatolius (-360)
- Florentius (360)
- Sextus Claudius Petronius Probus
- Quintus Clodius Hermogenianus Olybrius (378-379)
- Vettius Agorius Praetextatus (384, also Praetorian Prefect of Italy)
- Flavius Eutychianus (396-397)
- Anatolius (397-399)
- Herculius (408-410)
- Leontius (412-413)
- Flavius Iunius Quartus Palladius (416-421, also Praetorian Prefect of Italy)
- Gessius (some time between 421 and 443)
- Flavius Anthemius Isidorus (424)
- Flavius Simplicius Reginus (435)
- Eubulus (436)
- Thalassius (439)
- Apraeumius (441)
- Eulogius (c. 451)
- Valentinianus (452)
- Callicrates (468-469)
- Iohannes (472)
- Basilides (529)
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- Thessalonica, 1910 Catholic Encyclopedia
- Illyria, 1910 Catholic Encyclopedia
- It is a common mistake that the praetorian prefectures were established as territorial units by Constantine I already around 318 or 324, as anachronistically claimed by Zosimus. In reality, each Augustus or Caesar continued to have his own praetorian prefect as his chief of staff, and only by the mid-4th century did the prefectures become permanent administrative subdivisions of the Empire. Morrison (2007), pp. 190–191
- Morrison (2007), p. 396
- Greece, 1910 Catholic Encyclopedia
- Morrison (2007), pp. 397–398
- Southern Pannonia during the age of the Great Migrations
- Morrison (2007), pp. 401–402
- Bury (1912), pp. 223–224
- Notitia dignitatum
- Bury, John B. (1912), A history of the Eastern Roman empire from the fall of Irene to the accession of Basil I. (A. D. 802-867), London: Macmillan and Co.
- Kazhdan, Alexander, ed. (1991), Oxford Dictionary of Byzantium, Oxford University Press, ISBN 978-0-19-504652-6
- (Greek) Morrison, Cécile, ed. (2007), Le Monde Byzantin I - L'Empire romain d'orient (330-641) (in Greek), Athens: Polis Editions, ISBN 978-960-435-134-3
- * The Times History of Europe, Times Books, London, 2001.
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