For those of you interested in the rise of Islam I would recommend Henri Pirenne's book: Mohammed et Charlemagne. The Muslims, like Cortez, benefited a great deal from being in the right place at the right time. Byzantium had just exhausted itself by recapturing a large part of the Occident and fighting against the Persians. Local and religious politics also played a role as well. I'm not trying to take anything away from the Arabs mind you. The rise of Islam was also the catalyst for the onset of the Dark Ages in Europe when they closed the Mediterranean. Something which didn't affect the Italian traders too much.