The subdivisions of the Byzantine Empire were administrative units of the Eastern Roman or Byzantine Empire (330–1453). The Empire had a developed administrative system, which can be divided into three major periods: the late Roman/early Byzantine, which was a continuation and evolution of the system begun by the emperors Diocletian and Constantine the Great, which gradually evolved into the middle Byzantine, where the theme system predominated alongside a restructured central bureaucracy, and the late Byzantine, where the structure was more varied and decentralized and where feudal elements appeared.
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👉 Byzantine provinces in the context of Slavic migrations to the Balkans
Early Slavs began mass migrating to Southeastern Europe between the first half of the 6th and 7th century in the Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic.
The settlement was facilitated by the substantial decrease of the Southeastern European population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the steppe nomads against the Eastern Roman Empire. After the arrival of the Pannonian Avars in the mid-6th century, they continued to conduct incursions into Roman territory, often independently of Avar's influence. After the failed siege of Constantinople in the summer of 626, and successful revolt against the Avars, they remained in the wider Southeast Europe area after they had settled the Byzantine provinces south of the Sava and Danube rivers, from the Adriatic Sea to the Aegean and Black Sea.