Pirenne, Henri: The Expansion of Islam in the Mediterranean Basin 
From H.Pirenne’s «Mohammed & Charlemagne», Dover Publications, INC. Mineola, New York.
3. Venice and Byzantium
It may be said that the Islamic invasion was as decisive for the East as for the West of Europe. Before this invasion the Emperor of Constantinople was still the Roman Emperor. The policy of Justinian in this respect is characteristic: he claimed that the entire Mediterranean was subject to the Imperial authority. After the invasion, on the contrary, the Emperor was reduced to the defensive in Greek waters, until in the 11th century he appealed to the West for assistance. Islam immobilized and engrossed him. Here we have the whole explanation of his policy. Henceforth the Occident was closed to him.
Once Africa and Carthage were lost, after an obstinate defence under disastrous conditions, the sphere of action of Byzantine policy was confined to Italy; but even here only the coastal regions were retained. In the interior of the country Byzantium could no longer resist the Lombards. Her impotence led to the revolt of the Italians and the defection of the Pope. Henceforth the Empire strove to retain only Sicily, the Adriatic, and the cities of the South, but these outposts of Byzantium were becoming more and more autonomous.
The expansion of Islam came to a halt on the Byzantine frontier. Islam had robbed the Empire of its Syrian, Egyptian and African provinces, to some extent by exploiting national differences. But the Greek nucleus resisted, and by resisting it saved Europe, and doubtless, in saving Europe, it saved Christianity.
But the encounter was tremendous. Byzantium, twice attacked while Islam was at the height of its vigour, owed her victory to her fleet. She remained, in spite of all, a great naval power.
Of all the westward prolongations of the Byzantine Empire, the most important and the most original was the extraordinary city of Venice, the story of which, if we except the United Provinces, constitutes the most curious chapter in the economic history of the ages. The first inhabitants of the sandy and desolate islands of the lagoon were wretched fugitives, escaping from the hordes of Attila in the 5th century, at the time of the attack upon Aquileia. Other fugitives came to join them at the time of the Frankish occupation of Istria in the time of Narses,(60) and still more on the occasion of the Lombard invasion. The islands were thus populated by an exodus which was at first only temporary, but then became permanent. Grado received the greater number of the fugitives from Aquileia, whose bishop assumed the tide of patriarch, and became the spiritual head of the new Venetia. Caorle, in the estuary of the Livenza, was peopled by emigrants from Concordia, who were accompanied by their bishop. Next Heracliana was founded, and Aquileia by the Piave. The people of Altinum took refuge on Torcello, Murano, and Mazzorbo. The refugees from Padua settled at Malamocco and Chioggia. In the beginning the group of islands from which Venice was afterwards to rise were the most sparsely populated: Rialto, Olivolo, Spinalunga, Dorsoduro were occupied only by a few fishers.(61)