The seaboard of Africa lay open to the naval power of the Byzantine empire; but for a time, it was little used against the Saracens. Amru, with the restless spirit of his faith, soon pushed his conquests westward beyond the limits of Egypt, established himself in Barca, and reached even to Tripoli.[378] The subject races in these quarters rendered their tribute in a fixed quota of African slaves, thus early legalising in that unhappy land the iniquitous traffic which has ever since prevailed in human flesh and blood. The maritime settlements and native tribes thus ravaged, received little or no aid from the Byzantine fleets. But early in the Caliphate of Othmân, a desperate attempt was made to regain possession of Alexandria. The Moslems, busy with their conquests elsewhere, had left the city insufficiently protected. The Greek inhabitants conspired with the Court; and a fleet of three hundred ships was sent under command of Manuel, who drove out the garrison and took possession of the city. Amru hastened to its rescue. A great battle was fought outside the walls: the Greeks were defeated, and the unhappy town was subjected to the miseries of a second and a longer siege. It was at last taken by storm and given up to plunder. To obviate the possibility of another similar mishap, Amru razed the fortifications, and quartered in the vicinity a strong garrison, which, every six months, was relieved from Upper Egypt. The city, though still maintaining its commercial import, fell now from its high estate. The pomp and circumstance of the Moslem Court were transferred to Fostât, and Alexandria ceased to be the capital of Egypt.[379] CHAPTER XXV. ADVANCE ON THE SOUTHERN BORDER OF PERSIA--HORMUZAN TAKEN PRISONER. A.H. XVI.-XX. A.D. 637–641. [Sidenote: Barrier laid down by Omar towards the East.] Turning once more to the eastern provinces of the Caliphate, we find the cautious policy of Omar still tending to restrain the Moslem arms within the limits of Irâc-Araby; that is, within the country bounded by the western slopes of the great range which separates Chaldæa from Persia proper. But they were soon, by the force of events, to burst the barrier. [Sidenote: Situation in Lower Irâc.] To the north of Medâin, the border land of Moslem territory was securely defended by Holwân and other strongholds, already mentioned as planted along the hilly range. In Lower Irâc, Otba, as we have seen, had, after repeated encounters, established himself at Bussorah, from whence he held securely the country at the head of the Gulf.[380] But the Persian satraps, though keeping at a safe distance aloof, were still in strength at Ahwâz and Râm Hormuz within a hundred miles of him. [Sidenote: The Governor of Bahrein attacks Persepolis. A.H. XVI. A.D. 637.]