justinians plague: the plague killed 1/5th of the population of its imperial city
invading forces: The slavs attacked the Byzantium, so the Byzantines arranged for the avars to attack the slavs, which failed since it caused slavs to move further in the empire and the avars attacking the Byzantines
crusades: the Crusaders looted and destroyed parts of Constantinople
Ottomans: the Byzantine Ottoman wars led to the Byzantiums losing a lot of land and resources, even losing Constantinople by the hands of Sultān Mehmet Fatïh (Rahimullah)
