It is astonishing that more evils did not come upon the State during the boyhood of Constantinus. The energetic caliph Omar was still urging on the Arabs to conquest, and with no firm hand at the helm it might have been expected that the ship of the East-Roman state would have run upon the breakers. But though the Saracens still continued to make way, the rate of their progress was checked. Alexandria, the last Christian stronghold in Egypt, had fallen during the short reign of Heracleonas. The resources of the empire were drained for an attempt to recover it, and in the second year of Constantinus a considerable expedition, under a general named Manuel, fell unexpectedly upon the place and retook it. The Arab governor of Egypt, the celebrated Amrou, had to besiege the place for more than a year before it yielded. Irritated by its long resistance he cast down its walls and massacred many of its inhabitants. It would seem that the Saracen arms were for the next few years more engrossed in the final conquest of eastern Persia than in assaulting the Roman empire. It was not till Yezdigerd, the last of the Sassanian kings, had been defeated and stripped of the farthest corners of his dominion that the Arabs turned once more to the West.[38] Footnote 38: The final subjection of Persia was not complete till 652, though the battle of Nehavend, the last which Yezdigerd risked in the open field, was in 641. [Sidenote: War in Africa.] The only point of the Roman frontier which was seriously attacked was Africa. The sandy waste between Egypt and Barca had less terrors for the Arab than for any other invader. Encouraged by the fact that Gregory the exarch of Africa had rebelled and proclaimed himself emperor, so that he could hope for no aid from Constantinople, the Saracen general, Abdallah Abu-Sahr crossed the Libyan desert and attacked Barca. Gregory came out against him, but was defeated and slain: Barca and Tripoli fell to the invaders, but Carthage and the rest of Africa relapsed into allegiance to Constantinus, when the usurper was slain. The Saracen frontier stood still at the Syrtes (646-7), and it took half a century more of fighting before the Romans were evicted from the western half of their African possessions. Meanwhile the caliph Omar had died, and his weaker successor, Othman, proved less dangerous to the Eastern empire. His generals, however, invaded Cyprus, and overran the island: unable to permanently hold it, because of the preponderance of the Byzantine fleet, they contented themselves with exacting a tribute, and retired (642). But encouraged by the result of this, their first expedition by sea, the Saracens commenced to build a great war fleet, and in a few years they were in a condition to dispute the command of the eastern Mediterranean with the Roman galleys, who since the destruction of the Vandals in 533 had known no rivals on the sea.