 Consequently, they were vulnerable to the sudden emergence of the Islamic Rashidun Caliphate in Arabia; the Caliphate's forces invaded both empires only a few years after the war.The Muslim, also known as Arab, forces swiftly conquered the entire Sasanian Empire and deprived the Byzantine Empire of its territories in the Levant, the Caucasus, Egypt, and North Africa.By 642, the Muslim armies had conquered all of Syria and Egypt, the richest parts of the Byzantine Empire.