Two external forces served to shape the Middle Ages: the Byzantine Empire and Islam. More than merely defining the geographical borders of the medieval world, they interacted with it, producing, at times, friction and, at other times, great achievements. As much as any two individuals can effect movements and institutions containing their own inner dynamics, the emperor Justinian and the prophet Mohammed can be said to have shaped the Middle Ages by establishing contexts, limits and opposition to the European West. It was once said (by Henri Pirenne) that ‘without Mohammed Charlemagne would have been inconceivable’. Taking a broader view, it may be said that without Justinian and Mohammed there would never have been a Frederick II and Innocent III, i.e., there never would have been a medieval empire and a medieval papacy. It is a proposition worth examining.