William Muir,
The Caliphate: Its Rise, Decline and Fall…
“ Wherever Arabs went they formed a class apart and dominant,—the nobles and rulers of the land. Subject peoples, even if they embraced Islam, were of a lower caste; they could aspire to nothing higher than, as "clients" of some Arab chief or tribe, to court patronage and protection. Thus the Arabians set themselves apart, as a nation militant, for the sacred task of propagating Islam. Even after the 150new-born zeal of the Faith had evaporated, the chivalry of the Arabs as a race wholly devoted to arms, was, owing mainly to 'Omar's foresight, maintained in full for two centuries and a half. ”
