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By the time of the Islamic Conquests the Byzantine Empire was undoubtedly a naval empire at this point and as such Egypt and its North African holdings were key to feeding the military forces it used to keep its empire together on sea and land. What if the Byzantines were able to retain Egypt or at least recapture it at some point early on (maybe a successful Battle of Masts leads to a counter attack)?
Obviously one of the notable lead ups I think was a Haraclius' less than stellar administrative and religious choices for the province. Could a different compromise have lead to more local support by the Coptics?