By 613 the prophet Mohammed was preaching a new religion he called Islam. Largely ignored in his home city of Mecca, he withdrew to Medina, built up a strong following there, and returned to attack and capture Mecca. Following his death in 632, his teachings were collected to form the Koran, the Islamic holy book. In 634 his followers began their jihad, or holy war. Within five years they had overrun Egypt, Palestine, and Syria. Their tolerance of Jews and Christians eased their conquest because these people had been suffering some persecution under the Byzantines.

In the next 60 years, North Africa to the west and Persia to the east both fell to Islam. In the early 8th century, Saracens from Tangiers invaded Spain and conquered the Visigoth kingdom established there after the fall of Rome. In Asia they took Asia Minor from the Byzantines and attempted to capture Constantinople with a combined attack from land and sea. The great walls of the city frustrated the land attack and the Saracen fleet was defeated at sea. In the west, Charles Martel of the Franks stopped a Saracen invasion of modern France in 732 at Tours.

Frustrated in the west, the forces of Islam turned east. By 750 they had conquered to the Indus River and north over India into Central Asia to the borders of China.

In 656 the Muslim world fell into civil war between two factions, the Sunnites and the Shiites. They differed on several points, including who should be caliph and interpretation of the Koran. The result of the 60-year war was that the Islamic state broke into pieces, some governed by Sunnites (Spain) and others by Shiites (Egypt, modern Iraq). The new Islamic states acted independently, thereafter.

Muslim Spain developed into one of the great states of Europe during the early Middle Ages. Muslims, Jews, and Christians lived together in relative harmony and a rich culture rose out of these multiple influences. There was a flowering of the arts, architecture, and learning. By 1000, however, Muslim Spain had divided into warring factions. This civil war allowed the tiny Christian states of Castile and Aragon to begin the slow reconquest of the peninsula (the Reconquista), completed finally in 1492.

Asia Minor and the Middle East were conquered by Muslim Turks in the early 11th century. The Turks were much less tolerant of Christian pilgrims to Palestine and travel in the area became very dangerous. In response to a call for aid from the Byzantines, a series of crusades were launched from Europe to regain Palestine from the Turks. The independent Muslim states in the area lost Palestine and the Eastern Mediterranean coast to the First Crusade. In the last part of the 12th century the great Saracen leader Saladin succeeded in uniting Egypt, Syria, and smaller states, and he retook Jerusalem.