Great changes in the provincial government were introduced by Diocletian. During his reign the distinction between senatorial and imperial provinces disappeared; all provinces were dependent directly upon the emperor. Formerly, the provinces being comparatively few and territorially very large, their governors had enormous power in their hands. This condition had created many dangerous situations for the central government; revolts were frequent and the governors of these large provinces, supported by their legions, were often serious pretenders to the imperial throne. Diocletian, wishing to do away with the political menace of the large provinces, decided to divide them into smaller units.
The fifty-seven provinces in existence at the time of his ascension were divided into ninety-six new ones, perhaps more. Moreover, these provinces were placed under governors whose powers were purely civil. The exact number of smaller provinces created by Diocletian is not known because of the unsatisfactory information given by the sources. The main source on the provincial structure of the Empire at this time is the so-called Notitia dignitatum, an official list of court, civil, and military offices, which contains also a list of provinces. According to scholarly investigations, this undated document refers to the first half of the fifth century and hence includes the changes in provincial government introduced by the successors of Diocletian. The Notitia dignitatum numbers 120 provinces. Other lists, also of doubtful but earlier dates, give a smaller number of provinces.
Under Diocletian also a certain number of small new neighboring provinces were grouped together in a unit called a diocese under the control of an official whose powers were likewise purely civil. There were thirteen dioceses. In their extent the dioceses resembled the old provinces. Finally, in the course of the fourth century the dioceses in turn were grouped into four (at times three) vast units (prefectures) under praetorian prefects, the most important officials of that time. Since Constantine had shorn them of their military functions, they stood at the head of the whole civil administration and controlled both the diocesan and the provincial governors. Toward the end of the fourth century the Empire, for purposes of civil government, was divided into four great sections (prefectures): (1) Gaul, including Britain, Gaul, Spain, and the northwestern corner of Africa; (2) Italy, including Africa, Italy, the provinces between the Alps and the Danube, and the northwestern portion of the Balkan peninsula; (3) Illyricum, the smallest of the prefectures, which embraced the provinces of Dacia, Macedonia, and Greece; and (4) the East, comprising the Asiatic territory, as well as Thrace in Europe in the north and Egypt in the south.
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