Control of the Mediterranean had been the key to wealth and power in the Western world of antiquity. It was the core of the Roman Empire, which made it into the ‘mare nostrum’ (‘our sea’). Upon the fall of Rome, the Eastern Empire under Byzantium inherited sway over the sea and struggled for centuries to maintain it against barbarian incursions and the onslaught of Islam. In Mohammed and Charlemagne, the Belgian historian Henri Pirenne identified a dramatic shift in the balance of power in the Mediterranean in the last half of the first millennium. He called it ‘the end of Mediterranean unity’ and he blamed the change on ‘the rapid and unexpected advance of Islam’. While his conclusions have been called into question by later historians, his description of a pronounced power swing toward the East remains indisputable. At the beginning of the second millennium, Islam and Byzantium still dominated the Mediterranean. However, according to the maritime historian A. R. Lewis, that Eastern hegemony was effectively brought to an end in the last half of the eleventh century and a new era of Western maritime ascendancy took its place. The rise of Norman sea power was the primary reason for that profound change.

The seminal event in this seismic shift on the medieval ‘middle sea’ was the seizure of Sicily by the Normans. Sicily, in the center of the Mediterranean astride the east–west shipping channels, had long been the cornerstone to its dominance.