The military tsunami of the Rashidun caliphate conquered half of the Byzantine Empire in the middle of the 7th century. In response, the Byzantines became furiously consumed by an abstruse doctrinal dispute over whether Christ had one (divine) or two (human and divine) natures. A four-year siege of Constantinople finally brought them to their senses. Emperor Constantine IV managed to lift the siege, resolve the religious schism and had just enough time to have both his brothers castrated before dying of dysentery. His other legacy was the reorganization of shrunken armed forces into five regional units (themata) quartered respectively at Nicaea, Ephesus, Amisus, Amorium and Attalia. The empire saw no immediate respite, beset by attacks from Bulgars, the Kievan Rus, and the Islamic caliphates. However, from 842, the Macedonian dynasty would achieve an Indian summer for the Byzantines, vanquishing the Bulgars and wresting control of the seas from the caliphates.
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