Pirenne, Henri: The Expansion of Islam in the Mediterranean Basin 
From H.Pirenne’s «Mohammed & Charlemagne», Dover Publications, INC. Mineola, New York.
2. The Closing of the Western Mediterranean
So long as the Mediterranean remained Christian, it was the Oriental navigation that maintained commercial intercourse with the Occident. Syria and Egypt were its two principal centres; and these two wealthy provinces were the very first to fall under the domination of Islam. It would obviously be an error to believe that this domination put an end to all economic activity. Although there was great confusion and disorder, and although many Syrians migrated to the Occident, we must not suppose that the economic machinery collapsed. Damascus had become the first capital of the Caliphate. Spices were still imported, papyrus was still manufactured, the seaports were still active. Once they paid taxes to the conquerors, the Christians were not molested. Commerce, therefore, continued, but its direction was changed.(41)
It goes without saying that when an actual war was in progress the conqueror did not allow his subjects to trade with the conquered. And when peace had restored the commercial activity of the conquered provinces, Islam directed it into new trade routes, which were opened up by the immensity of its conquests.
These new trade routes connected the Caspian Sea with the Baltic, by way of the Volga, and the Scandinavians, whose merchants frequented the shores of the Black Sea, were suddenly compelled to follow the new route. Of this we need no further proof than the many Oriental coins found in Gothland.
We may be certain that the disturbances inseparable from the conquest of Syria (634-636) and then of Egypt (640-642) must have hindered navigation.(42)
The ships must have been requisitioned for the fleet which Islam suddenly organized in the Aegean Sea. Moreover, one can hardly imagine that merchants would have sailed through the midst of the hostile fleet, unless to take advantage of the situation, as many of them must have done, by turning pirate.
We may certainly conclude that from the middle of the 7th century navigation between the Musulman ports of the Aegean Sea and those ports which had remained Christian had become impossible; or if there was any, it must have been almost negligible.
It may have been possible to maintain navigation, under the protection of the fleet, between Byzantium and the adjacent coast, still defended by the capital, and the other Greek regions of Greece, the Adriatic, Southern Italy, and Sicily, but we can hardly suppose that the Byzantine vessels could have sailed any farther, since Islam was attacking Sicily as early as 650.