After the death of Charlemagne, king of the Franks, in 814 and the following collapse of his empire, Christian Europe was under attack and on the defensive. The Magyars, nomadic people from Asia, ravaged eastern and central Europe until the 10th century. Around 800, several centuries of Viking raids disrupted life in northern Europe and even threatened Mediterranean cities. Nevertheless, the greatest threat came from the forces of Islam, very militant and victorious in the centuries following the death of their leader, Muhammad, in 632. By the eighth century Islamic forces had conquered North Africa, the eastern shores of the Mediterranean, and most of Spain. Islamic armies established bases in Italy, greatly reduced the size of the Byzantine Empire, and besieged the capital, Constantinople. In the 11th century the balance of power began to swing toward the West. For the first time in many years, the popes were able to effectively unite European popula...