It was a great matter of reproach against the caliph Othman that he was injudicious in his appointments, and had an inveterate propensity to consult the interests of his relatives and friends before that of the public.  One of his greatest errors in this respect was the removal of Amrou ben-el-Ass from the government of Egypt, and the appointment of his own foster-brother, Abdallah Ibn Saad, in his place.  This was the same Abdallah who, in acting as amanuensis to Mahomet, and writing down his revelations, had interpolated passages of his own, sometimes of a ludicrous nature.  For this and for his apostasy he had been pardoned by Mahomet at the solicitation of Othman, and had ever since acted with apparent zeal, his interest coinciding with his duty.

He was of a courageous spirit, and one of the most expert horsemen of Arabia; but what might have fitted him to command a horde of the desert was insufficient for the government of a conquered province.  He was new and inexperienced in his present situation; whereas Amru had distinguished himself as a legislator as well as a conqueror, and had already won the affections of the Egyptians by his attention to their interests, and his respect for their customs and habitudes.  His dismission was, therefore, resented by the people, and a disposition was manifested to revolt against the new governor.

The emperor Constantine, who had succeeded to his father Heraclius, hastened to take advantage of these circumstances.  A fleet and army were sent against Alexandria under a prefect named Manuel.  The Greeks in the city secretly cooperated with him, and the metropolis was, partly by force of arms, partly by treachery, recaptured by the imperialists without much bloodshed.

Othman, made painfully sensible of the error he had committed, hastened to revoke the appointment of his foster-brother, and reinstated Amru in the command in Egypt.  That able general went instantly against Alexandria with an army, in which were many Copts, irreconcilable enemies of the Greeks.  Among these was the traitor Mokawkas, who, from his knowledge of the country and his influence among its inhabitants, was able to procure abundant supplies for the army.

The Greek garrison defended the city bravely and obstinately.  Amru, enraged at having thus again to lay siege to a place which he had twice already taken, swore, by Allah, that if he should master it a third time, he would render it as easy of access as a brothel.  He kept his word, for when he took the city he threw down the walls and demolished all the fortifications.  He was merciful, however, to the inhabitants, and checked the fury of the Saracens, who were slaughtering all they met.  A mosque was afterward erected on the spot at which he stayed the carnage, called the Mosque of Mercy.  Manuel, the Greek general, found it expedient to embark with all speed with such of his troops as he could save, and make sail for Constantinople.