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What if through the power of hand-wavium (longer internal strife between Theodosius III and Leo III, Christian crewman of Arab navy don't defect) Constantinople falls in 718AD? The Arabs still are defeated during the Battle of Tours, but the gate to Europe through the Balkans is open. What happens? Does Russia convert to Islam? How far does Islam go? How does this change the Mongol invasions centuries later?
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