Following the successive administrative reforms of Diocletian and Constantine the Great, the province of Macedonia ('provincia Macedonia') in the 4th century comprised part of the administrative area that was known by the same name, and was subject to the prefecture of Illyricum ('praefectura praetorio per Illiricum').
The Roman Empire
After 337 AD
Early in the 5th century the territory of Illyricum was divided into two, of which the eastern part (the dioceses -- 'dioeceses' -- of Macedonia and Dacia) was ceded to the Eastern Roman state. The diocese of Macedonia, which included Thessalonike, the seat of the prefecture of Illyricum, was of great political significance on account of its strategic position. For this reason it was reinforced in the 6th century with fortresses erected at vital points.