It was the Western Anatolia that was not under the control of Arab after the death of Muhammad within 10 years. Western Anatolia was under the control of Byzantine. The Byzantine were mostly Greek speaking people and that’s why Anatolia were also Greek speaking. Anatolia was traditionally considered as the extension in the East to the line between Gulf of Iskenderun up to the black sea. Despite the under ruling problem of the Byzantine, they were still able to hold Anatolia.
Within a decade after Muhammad's death, most of the Middle East came under Arab control, except for Western Anatolia. This region remained under Byzantine control due to their military strength and strategic defenses. The Byzantines' ability to hold onto Anatolia allowed them to retain a significant presence in the eastern Mediterranean for centuries.
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