In the Late Roman Empire, usually dated AD 284 to 641, the regional governance district known as the Roman or civil diocese was made up of a grouping of provinces each headed by a vicarius, the vicars being the representatives of praetorian prefects (who governed directly the dioceses they were resident in). There were initially twelve dioceses, rising to fourteen by the end of the 4th century. The term diocese comes from the Latin dioecēsis, which derives from the Ancient Greek dioíkēsis (διοίκησις) meaning 'administration', 'management', 'assize district', or 'group of provinces'.
Historical development
Tetrarchy (286–305) Two major reforms to the administrative divisions of the empire were undertaken during the Tetrarchy. The first of these was the multiplication of the number of provinces, which had remained largely unchanged since the time of Augustus, from 48 at the beginning of Diocletian's reign to around a hundred by the time of his abdication. The multiplication of the provinces was probably undertaken for military, financial, and economic reasons. It brought the governor closer to the cities which were responsible for the collection of taxes. It also limited the power of the governors and the autonomy of the cities. At the same time, the status of the provinces was regularized. Egypt lost its unique status and was divided into three provinces, while Italia was 'provincialized' – the numbered regiones established by Augustus received names and were governed by correctores. The distinction between senatorial and imperial provinces was abolished and henceforth all governors were appointed by the Emperor. In order to compensate for the weakening of the provinces and to maintain the link between the Imperial centre and the individual provinces, the dioceses were created as a new territorial subdivision above the level of the province. The empire was divided into twelve dioceses. The largest of these, the Diocese of the East, encompassed sixteen provinces. Each diocese was governed by an agens vices praefectorum praetorio (Acting Representatives of the Praetorian Prefects) or simply Vicar (vicarius), under the Praetorian prefect, although some provinces were governed directly by the Praetorian Prefect. These vicars had previously been ad hoc representatives of the prefects, but they were now made into permanent, regularised positions. The vicar controlled the provincial governors (variously titled as consulares, correctores, praesides) and heard appeals of cases decided at the provincial level (parties could decide whether to appeal to the vicar or the praetorian prefect). The provinces governed by proconsuls (Africa and Asia) remained outside the vicars' jurisdiction,. as did the cities of Rome and Constantinople, which were governed by a Praefectus urbi instead. The vicars had no military powers. Troops stationed in the dioceses fell under the command of a comes rei militaris, who was directly under the control of the magister militum and was in charge of the duces who had the military command of individual provinces. Many modern scholars date the introduction of the dioceses to AD 296–297. A passage of Lactantius, who was hostile to Diocletian because of his persecution of the Christians, seems to indicate the existence of vicarii praefectorum in the time of Diocletian:
And so that everything would be filled with terror, the provinces were also sliced to bits; many governors and more offices brooded over individual regions and almost every city, as well as many rationales, magistri, and vicarii praefectorum, all of whose civil acts were exceedingly rare, but whose condemnations and proscriptions were common, whose exactions of innumerable taxes were not so much frequent as constant, and the damage from these taxes was unbearable. Thus Lactantius refers to the vicarii praefectorum as being active already in Diocletian's time. Other sources from Diocletian's reign mention one Aurelius Agricolanus who was an agens vices praefectorum praetorio active in Hispania and condemned a centurion named Marcellus to be executed for his Christianity, as well as an Aemilianus Rusticianus, who is considered by some scholars to have been the first vicar of the Diocese of the East that we know of. Lactantius also mentions one Sossianus Hierocles as an ex vicario active in the East in this period. Septimius Valentio is also attested as agens vices praefectorum praetorio of Rome between 293 and 296. However, these sources do not prove that these vicarii or agentes vices were already in charge of dioceses with a well-defined and stable territory. Septimius Valentio in particular was definitely the commander of the Praetorian Guard during a period when the Praetorian Prefect was absent from the city, but was not in charge of Italia Suburbicaria. According to Zuckerman, the establishment of the dioceses should instead be dated to around AD 313/14, after the annexation of Armenia into the Roman empire and the meeting of Constantine and Licinius in Mediolanum. The matter remains controversial.
Constantinian reforms (326–337)
From 310, Constantine I was one of the Augusti of the Empire and from 324 he was the sole ruler of the whole state. During his reign, he carried out many crucial reforms creating the administrative and military organization of the empire which would last until the fall of the Western Roman Empire.
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