One of four territorial divisions of the Late Roman Empire
The praetorian prefecture of the East, or of the Orient (, ) was one of four large praetorian prefectures into which the Late Roman Empire was divided. As it comprised the larger part of the Eastern Roman Empire, and its seat was at Constantinople, the praetorian prefect was the second most powerful man in the East, after the Emperor, in essence serving as his first minister.
Structure.
The Prefecture was established after the death of Constantine the Great in 337, when the empire was split up among his sons and Constantius II received the rule of the East, with a praetorian prefect as his chief aide. The part allotted to Constantius encompassed four (later five) dioceses, each in turn comprising several provinces. The authority of the prefecture stretched from the Eastern Balkans, grouped into the Diocese of Thrace, to Asia Minor, divided into the dioceses of Asiana and Pontus, and the Middle East, with the dioceses of Orient and Egypt.
References.
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