It started during the Umayyad period, when newly-conquered peoples were set free after conversion or after accepting a patron-client relationship with one of the ruling overlords. Typical story of this were the Persians: "a new element in Muslim society, noble and refined, but held apart and altogether inferior to rude but dominant class of Arabian blood. Individuals or families belonging to the subject peoples could only gain a recognised position by attaching themselves to some Arab chief or clan, as mawali (plural of maula), "clients" or adherents; and, though thus dependent, might claim some of the privileges of the ruling faith. But neither here nor elsewhere did they intermarry with the Arabs on equal terms, nor were they, in point of fact, looked upon otherwise than as of inferior caste. Thus though in theory, on becoming Muslims, conquered nations might enter the "equal brotherhood" of Islam, they formed not the less an altogether lower estate. The race and language, ancestral dignity, and political privileges of the Arab hue continued for many generations to be paramount."


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Note added at 2003-04-25 00:28:05 (GMT)
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An interesting link: http://www.fordham.edu/halsall/med/schacht.html