In Alexandria there was a considerable population of the Byzantines who owed nominal allegiance to the Muslims, and whose loyalties lay with the Government of Byzantium.
With the death of Umar, and the deposition of 'Amr b Al 'Aas from the Governorship of Egypt of Byzantines of Alexandria felt that it was the right moment to throw off the yoke of the Muslims. The Byzantines of Alexandria moved the Emperor of Byzantium, Constans to invade Egypt and reconquer it from the Muslims.
Early in 646 C.E., a large Byzantine force landed at Alexandria. There was only a small Muslim garrison in Alexandria generally remained faithful to the Muslims, but they were not in a position to offer any resistance to the Byzantines. The Byzantine forces occupied Alexandria without much difficulty.