AT THE CROSSROADS OF CIVILIZATION
1. The Arab Conquests: Opening or Closure
 The global impact of the Islamic conquests has been an issue of historical debate since Henri Pirenne formulated the problem a half century ago. In Pirenne's view, the conquest of the eastern and southern shores of the Mediterranean, of Spain, and of strategic islands had shut off the mainsprings of the movement of world trade which had flourished during the late Roman times, with the result that western Europe felt an intensification of ruralization and was impelled to return to a closed, moneyless, "natural" economic system. The conquests, then, set in motion a chain of events that was, centuries later, to result in the shifting of the balance of power in Europe from the Mediterranean region northward.
In fact, the Islamic conquest had more nearly the opposite effect than that posited by Pirenne: it opened the Mediterranean, previously a Roman lake, and, by connecting it with the Indian Ocean, converted it into a route of world trade.(1) Initially, there was no dislocation of the international economic system and, in the 690's when 'Abd al-Malik tried an economic blockade against the Byzantine Empire, only a limited and partial closure was achieved: only the eastern Mediterranean was affected, and although the flow of certain items, such as papyrus, was interdicted, other products, such as spices, traveled as before. When Byzantine power reasserted itself, between 752 and 827, it was the Byzantines who closed off trade, not the Arabs. During this period there was indeed a retreat from the Mediterranean, but a double retreat: the Franks to Germany, the Arabs to Iraq. Spain, it appears, was minimally affected by this situation, owing to a tacit alliance of the Umayyad Emirate with the Byzantine Empire in mutual opposition to the Franks.
The result of Byzantine-Arab confrontation was to throw the former into a situation of economic dependence on western Europe for the raw materials it could no longer obtain from the East and to make the West a market for Byzantine goods. This was a reversal of the economic balance of Roman times, when the West had been dependent on the East. By the tenth century, when the Muslims had taken control of strategically  important islands (Crete, Sicily, the Balearics) Islam effectively controlled the Mediterranean, which did not constitute a barrier to trade, but rather a medium whereby all bordering states could participate in a world economy, fertilized by healthy injections of Sudanese gold.