century: the sea-borne transport of commodities is shown by the state-directed supply of corn from Egypt to Constantinople and by the discovery of a wreck off south-west Turkey which sank around 623 with a cargo of wine in amphorae and other mass-produced utensils. In the western ( Tyrrhenian) basin, piracy and the activities of the Vandal fleet in the fifth century dealt long-distance traffic a blow from which it never recovered, even though short coastal voyages continued because they were cheaper and safer than overland communication. Africa's trade in foodstuffs declined into insignificance. Justinian's defeat of the Vandals and Goths only partially restored Roman naval power. Even in the Aegean area the Byzantine naval monopoly was broken when the Ummayads built up a fleet composed largely of Christian renegades, seized Christian bases such as Cyprus and Rhodes, and twice blockaded Constantinople itself.

The Byzantines were quick to appreciate the importance of naval supremacy in defending and provisioning their many coastal provinces, and retrieved the situation by building new 'thematic' fleets supplied from particular areas. This naval recovery was helped by the rise of the Abbasid caliphs based on Baghdad, who were less sea-minded than their Ummayad predecessors. In the ninth century Byzantium's naval hegemony was undermined by a series of revolts in the naval themes and by the emergence of aggressive new Muslim states, especially the Aghlabids of North Africa. As a result of this Sicily was invaded in 827, Crete was lost in 828, and the empire's western provinces suffered devastating raids. Although Byzantine naval dominance was therefore seriously challenged from around 650, the empire retained a strong presence in those waters which were strategic to its survival, thanks to the technical superiority of its ships, the professionalism of its sailors, and the advantage of plentiful timber and naval supplies in comparison with the Arabs. As for communications with the west, the Arabs only seriously threatened Byzantium's control of the middle Mediterranean ( Adriatic) basin in the ninth century, when her subject cities of Venice and Amalfi were developing a powerful naval capability.

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