Unlike the other three classical prefectures listed in the
Notitia Dignitatum—
Gaul, the
Italy–Africa and the
East—the fourth-century history of Illyricum as a prefecture involved abolition, re-establishment, and repeated division. Later writers created the impression that Constantine I established territorial prefectures early in the fourth century, but contemporary practice kept the
praetorian prefect as the emperor's chief of staff, and only by the mid-fourth century did the prefectures become enduring territorial units. The dioceses of
Macedonia,
Dacia, and
Pannonia were probably first grouped together in 347 by
Constans, when they were detached from the Italian prefecture. Some scholars prefer an earlier stage in 343, when Constans appointed a separate prefect for Italy, which implies a corresponding rebalancing in the Danubian and Balkan provinces. During 384–395 the two eastern dioceses were once more attached to the Italian prefecture, except for 388–391, when they again formed a separate Illyrian prefecture. The pressures of the
Byzantine–Bulgarian Wars led to reorganization. By the early ninth century
Thessalonica formed a distinct theme under a
strategos and the old prefectural framework had vanished from practical administration. ==List of known
praefecti praetorio per Illyricum==