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Your challenge is to make a huge part of the Balkans be conquered by the Arabs during their expansion between 700 and 800 AD (Constantinople included) but be completely recovered by the Christians before 1500 in a long process of "Reconquista". In the other hand, the Iberian Peninsula must be the target of a later Muslim expansion starting in the 1300's and a part of it must still belong to a Muslim country nowadays. How could it be done?