In Morocco by Edith Wharton — Read

The first Arab invasion of Morocco is said to have reached the Atlantic coast, but it left no lasting traces, and the real Islamisation of Barbary did not happen till near the end of the eighth century, when a descendant of Ali, driven from Mesopotamia by the Caliphate, reached the mountains above Volubilis and there founded an empire. The Berbers, though indifferent in religious matters, had always, from a spirit of independence, tended to heresy and schism. Under the rule of Christian Rome they had been Donatists, as M. Bernard puts it, "out of opposition to the Empire"; and so, out of opposition to the Caliphate, they took up the cause of one Moslem schismatic after another. Their great popular movements have always had a religious basis, or perhaps it would be truer to say, a religious pretext, for they have been in reality the partly moral, partly envious revolt of hungry and ascetic warrior tribes against the fatness and corruption of the "cities of the plain."

Idriss I became the first national saint and ruler of Morocco. His rule extended throughout northern Morocco, and his son, Idriss II, attacking a Berber tribe on the banks of the Oued Fez, routed them, took possession of their oasis and founded the city of Fez. Thither came schismatic refugees from Kairouan and Moors from Andalusia. The Islamite Empire of Morocco was founded, and Idriss II has become the legendary ancestor of all its subsequent rulers.