Attachment 124184
This is the map of the Abbasid Empire at its height c. 850 CE. As you can see, Most of the Middle East and eastern parts of Anatolia were already under Muslim control before the Turks came in 1071 AD. The main reason of decay of the Byzantine Empire was their long time struggle against Sasanids. The continuous struggle exhausted both empires, leaving them vulnerable. Seizing the opportunity, the Arabs capitalized on their weakened state and successfully conquered both territories.