Roman diocese
The year 296 marked a turning point when Emperor Diocletian began reshaping the Roman Empire. He multiplied the number of provinces from forty-eight to around one hundred during his reign. This change brought governors closer to cities responsible for tax collection. It also limited the power of those governors and reduced city autonomy. Egypt lost its unique status and was divided into three provinces. Italia received names for its numbered regiones and were governed by correctores. The distinction between senatorial and imperial provinces disappeared, making all governors appointments of the Emperor. To compensate for weaker provinces, dioceses emerged as new territorial subdivisions above the provincial level. Twelve dioceses covered the empire initially. The largest, the Diocese of the East, encompassed sixteen provinces. Each diocese was governed by an agens vices praefectorum praetorio or simply Vicar under the Praetorian prefect. Some provinces remained directly under the Praetorian Prefect. These vicars had previously been ad hoc representatives but became permanent positions. They controlled provincial governors and heard appeals decided at the provincial level. Troops in the diocoses fell under the command of a comes rei militaris who reported to the magister militum. Many modern scholars date the introduction of these dioceses to AD 296, 297.
Between 326 and 337 Constantine I transformed the ministerial Praetorian Prefect into a regional Prefect. A single emperor could not control everything so he created specific territories called Praetorian Prefectures containing several dioceses. This administrative decentralization reduced the utility of the dioceses themselves. The direct link between prefects and governors bypassed the Vicars causing their power to decline. They increasingly became agents carrying out the will of the Praetorian Prefects. Despite this decreased importance the vicars played an important role in court hierarchy. Constantine raised them to the rank of clarissimi between consulares and proconsulares. Another reason for weakening the vicars was the regular dispatch of comites who outranked them. These officials probably inspected their conduct. In 312 Constantine likely transformed the agens vices prefectorum praetorio of Rome into the civil vicar of Italia Suburbicaria. He split the diocese of Italia into two vicariates: Italia Suburbicaria in the south and Italia Annonaria in the north. Under Valens between 364 and 378 the Diocese of Egypt separated from the Diocese of the East. The Notitia Dignitatum indicates that at some point the Diocese of Gaul was suppressed and incorporated into the diocese of the Septem Provinciae.
In 535, 536 Emperor Justinian decided to abolish the dioceses of the East Asia and Pontus. Their vicars were demoted to simple provincial governors. For example the comes Orientis became governor of Syria I while vicars of Asia and Pontus became governors of Phrygia Pacatiana and Galatia I respectively. In May 535 Justinian abolished the vicariates of Thrace and the Long Walls to improve defense by ending conflicts between the two vicars. He entrusted administration of the diocese of Thrace to a praetor Iustinianus with civilian and military powers. A year later he introduced a new prefecture called the Prefecture of the Islands governed by a quaestor exercitus based in Odessa. This prefecture contained provinces like Moesia II Scythia Minor Insulae Caria and Cyprus. In 539 Justinian also abolished the diocese of Egypt splitting it into five independent circumscriptions governed by duces with civilian and military authority. The Prefect of Egypt was renamed dux augustalis leaving control over only Aegyptus I and Aegyptus II. These modifications aimed to end conflict between civilian and military officials moving away from Diocletian's principle of separating power. By abolishing the dioceses Justinian attempted to simplify bureaucracy and decrease state expenses since their courts of appeal were used less frequently. Thirteen years after these reforms in 548 Justinian decided to re-establish the diocese of Pontus due to serious internal problems.
A high official appointed by the Emperor and accountable only to him held the position of vicarius. The office was held by equites given the rank of perfectissimus before egregii and after eminentissimi. In rank vicars were inferior to governors of senatorial provinces though they exercised political authority over them. Constantine the Great raised them to the senatorial rank of clarissimus in 324, 325. Initially powers of the vicars were considerable as they controlled and monitored governors aside from proconsuls governing Asia and Africa. They administered tax collection and intervened in military affairs to fortify borders while judging appeals. They were not under control of the Praetorian Prefect but only to the Emperor himself. Appeals of their legal decisions went straight to the emperor. As responsible for integrity of global diocesan budgets drawn up by prefectures they received oversight powers in 328, 329 AD over Treasury and Crown Estate officials. They could not meddle in routine business yet processed huge amounts of fiscal and judicial work from provincial level before sending it to prefectures. Their offices resided in same towns or cities facilitating work of diocesan staffs auditing and processing information. The additional authority truly made vicars mini-prefects. The position declined from first decades of fifth century as emperors switched back to two tier arrangement rather than three level with diocese as regional level.
Around end of fifth century majority of Western Roman Empire dioceses ceased to exist following establishment of Barbarian kingdoms. There is no evidence that Franks and Burgundians maintained Roman provincial system. Visigoths and Vandals did maintain provinces governed by rectores or iudices but not dioceses or prefectures. In Italia Odoacer and then Ostrogothic kings particularly Theoderic retained Roman provincial system including Praetorian Prefecture of Italia and two vicariates of Italia Annonaria and Italia Suburbicaria. When Theoderic conquered Provence in 508 he re-established a Diocese of the Gauls promoted to rank of Prefecture with capital at Arelate two years later. This Praetorian Prefecture was abolished in 536 during reign of Vitiges after cession of Provence to Franks. Rationale behind Odoacer and Theoderic maintenance of Roman system was that they were officially viceroys of Roman emperor in Constantinople for whom Italia nominally continued forming part of empire. Civilian offices including vicars praesides and Praetorian Prefects continued filled with Roman citizens while Barbarians without citizenship barred from holding them. According to Cassiodorus authority of vicarius urbis Romae diminished: in fourth century he controlled only land within forty miles of City of Rome instead of ten provinces.
In seventh century establishment of first themes led to final extinction of diocese system. These military districts governed by strategos held both military and civilian authority alongside invasions of Arabs and Slavs. Praetorian Prefectures of East and Illyricum disappeared as result. Last certain attestation of Praetorian Prefect of East is in 629 while Illyricum survived to end of seventh century but without effective power since majority Balkans aside from Thessaloniki fell under Slavs. Prefect of Illyricum renamed Praetorian Prefect of Thessaloniki. In same period dioceses of Dacia and Macedonia finally disappeared due to loss of almost all territory. Taktikon Uspenskij written beginning ninth century mentions Praetorian Prefect of Constantinople and Proconsuls suggesting Prefecture of East continued existing though losing earlier powers and having few judicial functions. If dioceses lost fiscal functions during sixth and seventh centuries they may have been replaced by new groupings of provinces under judicial administration of Proconsul. Provinces continued existing under themes until second half of ninth century. The vicars and other civilian officials seem to have lost most importance to exarchs and subordinates but did not disappear until middle of seventh century AD.
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Common questions
When did Emperor Diocletian establish the Roman diocese system?
Emperor Diocletian established the Roman diocese system in the year 296. This reform multiplied provinces from forty-eight to around one hundred and created twelve initial dioceses covering the empire.
Who governed a Roman diocese under Diocletian's reforms?
Each Roman diocese was governed by an agens vices praefectorum praetorio or simply Vicar under the Praetorian prefect. These vicars controlled provincial governors and heard appeals decided at the provincial level.
What happened to Roman dioceses during the reign of Constantine I between 326 and 337?
Constantine I transformed the ministerial Praetorian Prefect into a regional Prefect creating specific territories called Praetorian Prefectures containing several dioceses. This administrative decentralization reduced the utility of the dioceses themselves and caused the power of vicars to decline.
How did Emperor Justinian change the Roman diocese system in 535 and 536?
In 535 and 536 Emperor Justinian abolished the dioceses of East Asia and Pontus while also splitting the diocese of Egypt into five independent circumscriptions. He aimed to simplify bureaucracy and decrease state expenses since their courts of appeal were used less frequently.
When did the Roman diocese system finally disappear in the seventh century?
The Roman diocese system disappeared in the seventh century following the establishment of first themes which held both military and civilian authority. The last certain attestation of the Praetorian Prefect of East is in 629 while most Balkans fell under Slavs before the end of that century.