The true precursors of the theme organization were the exarchates of Ravenna and Carthage (Africa), established at the end of the sixth century. The attacks of the Lombards caused the drastic change in the administration of Italy, as those of the Berbers (Moors) caused in North Africa. The central government, with a view toward creating a more efficient defense against its enemies, attempted to form large territorial units with strong military authorities in its border provinces. The Persian, and later the Arabian, conquests of the seventh century, which deprived the Byzantine Empire of its eastern provinces, completely changed conditions in Asia Minor. From a land which practically never needed any serious defense it became transformed into a territory constantly and strongly menaced by its Muslim neighbors.
The Byzantine government was forced to undertake decisive measures on its eastern border: military forces were regrouped and new administrative divisions were established, giving predominance to the military authorities, whose services at this time were of extreme importance. Equally great was the menace from the newly constructed Arabian fleet, which was almost master of the Mediterranean Sea as early as the seventh century, and threatened the shores of Asia Minor, the islands of the Archipelago, and even the shores of Italy and Sicily. In the northwest of the Empire the Slavs occupied a considerable part of the Balkan peninsula and penetrated far into Greece, including the Peloponnesus. On the northern border rose the Bulgarian kingdom (in the second half of the seventh century). These altered conditions forced the Empire to resort in the most insecure provinces to the establishment of extensive districts ruled by strong military power, similar to the exarchates. The Empire was militarized.
A History of the Byzantine Empire - Table of Contents
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