During the post classical era the Abbasid and Byzantine Empire suffered from internal and external threats. The crusades were a external threat to the empires. In 1202- 1204 the Byzantine was attacked during the fourth crusade. So did the Abbasid empire. However, the Abbasids were not weakened by these attacks while the Byzantine were greatly weakened. The Seljuk Turks and other threats led to the fall of these empires. In 1021 the Seljuk Turks weakened the Byzantine empire by defeating the Byzantine army. In 1055 the Selkjuk Turks attacked the Abbasid Empire. Then in 1258 the Mongols sacked Baghdad, the Abbasid's capital, resulting to the fall of the Abbasid empire. In 1453 the Byzantine empire fell to the hands of the Ottoman. The empires were weakened and eventually fell to outside invaders. In the societies of both empires there were internal revolts. These revolts weakened the empire. The revolts were caused by different people and reasons. In the Abbasid empire the peasants revolted because of hunger and high taxes. Over in the Byzantine empire there was the theme system. There was a fight for power between the powerful families. Although in the beginning the revolts began with these people it later included the peasants who were over pressured. So both empires had revolts which involved peasants. In conclusion, the Abbasid and Byzantine empire were very similar in decline and fall die to the external threats and internal problems. Although they both had their problems the invaders, reasons, and people involved in their decline and fall were not the same. (opinion >.