The origins and growth of Islam are indeed exceedingly difficult to explain; but the facts are not in dispute. Muhammad's preaching began soon after his earliest revelations in about 610 [C.E.]. In 622 he left Mecca for Medina, the famous Hijra or migration which has ever since featured as Year I in the Muslim calendar. By the time of his death in 632 the community which he had founded had come to embrace many of the tribes which inhabited the Arabian peninsula. A new power had been born. In the following generation the caliphs who succeeded to the Prophet's leadership — and the Arabic word khalifa means simply 'successor' — unleashed the military energies of the tribesmen upon the settled peoples of the Fertile Crescent. At that date the area was dominated by the two superpowers of the ancient world, the Persian and Roman empires. Between 633 and 651 Persia was defeated by Islam and overrun in a series of lightning campaigns: Islamic dominion in the East reached into modern Afghanistan. In the early seventh century the Roman empire had consisted of the eastern and southern Mediterranean lands stretching from Greece and the Balkans through Asia Minor, Syria, Palestine, Egypt and on through North Africa to distant outposts in modern Morocco. (It is often referred to as the Byzantine empire, a name derived from the settlement which underlay the empire's capital city, Constantinople, but its rulers referred to themselves as Roman emperors. The western provinces of the empire had been taken over by Germanic invaders at an earlier date.) Between 634 and 638 Palestine and Syria were conquered by the Islamic armies. Egypt followed in the years 640-2, and in the following year Arab forces began to stream into the provinces that form the modern state of Libya.
Thus within twenty years of Muhammad's death his followers had destroyed one ancient empire and hacked great chunks off another. Mighty cities such as Antioch and Alexandria had fallen into Muslim hands. The most sacred sites in Christendom, the Holy Places of Jerusalem and Palestine, had been lost: not for over four centuries would Christian armies attempt to recover them.
— Richard Fletcher, The Quest for El Cid, p. 11