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.
Recall of 'Amr b Al 'Aas
From the base at Alexandria, the Byzantines planned to reconquer the whole of Egypt. The Muslims of Egypt sent a delegation to Madina to wait on Uthman, and prevail on him to restore 'Amr b Al 'Aas to the command of Egypt for he alone could save Egypt for the Muslims.
Uthman summoned 'Amr b Al 'Aas , and appealed to him to take over the command of Egypt. He stipulated some terms which were accepted by Uthman. 'Amr b Al 'Aas was made the Governor of Upper as well as Lower Egypt, and he was also to be the supreme commander of the military forces in Egypt. Abdullah b Saad was to hold a subordinate position under 'Amr b Al 'Aas .