Simplify the following text for a Grade 13 native English-speaking person.
Original: In the east , partially resulting from the destructive Plague of Justinian , the Byzantines were threatened by the rise of Islam , whose followers rapidly conquered the territories of Syria , Armenia and Egypt during the Byzantine-Arab Wars , and soon presented a direct threat to Constantinople .
Simplified: The Byzantines were threatened by the rise of Islam , whose followers took over the territories of Syria , Armenia and Egypt and soon threatened to take over Constantinople .