Byzantium’s massive loss of territory to the Rashidun Caliphate in the 7th century had one substantial upside: its shrunken borders were much more defensible. The Byzantines adapted pragmatically to their reduced circumstances. The system of military and administrative divisions called themes was established by 700 CE and harkened back to the Roman Republic by fusing military and civil administration, with the soldiers of each theme receiving land for farming in return for service. Additional divisions, kleiosaurai, were created specifically to guard the mountain passes on the empire’s borders. This structure, cemented by a rigorous system of military training, made the Byzantine heartlands hard to penetrate, as defenders were protecting their own soil. In the end, the system was undermined by its own success. Under the Macedonian dynasty (867–1057), the Byzantines turned the tables on their Arab and Bulgar adversaries; the themes that administered the spate of new conquests were hastily created, undermining their effectiveness.
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