Don't forget the Persians before the Muslims or all the self-inflicted damage from coups, inept emperors, etc.. In this case though, the fourth crusade really

Don't forget the Persians before the Muslims or all the self-inflicted damage from coups, inept emperors, etc.. In this case though, the fourth crusade really is important since it was when the Hagia Sophia was first looted during a sack of the city. Nearly seven centuries of accumulated treasures were carted out of the place.