The Arabs who invaded Syria and Iraq in the 630s were not only the last and most successful of the barbarians to overrun the Roman frontier, but the last and greatest of the heretics. As followers of a new man ‘sent from God’ in the Biblical tradition of Judaism and Christianity, they differentiated themselves from both as the champions of a final revelation. And as his followers, they differed from the rest of the barbarians in the faith that turned them from tribesmen into the members of a disciplined community with a mission to conquer and rule the world. By the 640s they had appropriated the whole of the Persian empire together with the Roman provinces of Syria and Egypt, and were probing southwards into Nubia and westwards into North Africa. The conquest of North Africa was delayed until the end of the century, but was rapidly followed by the conquest of Spain. By 715, a hundred or so years after the date chosen for the beginning of their new era in 622 CE, they ruled an empire from the Atlantic to India and Central Asia from a capital at Damascus.

The conquest within a few years of Syria, Egypt, Iraq and Iran followed twenty years of exhausting warfare between the Persian and the Byzantine empires, in which Egypt had been occupied by the Persians for over ten. When the Byzantines returned, the province was only lightly garrisoned with the aid of local levies, while the population was alienated by renewed persecution of the Monophysites.