Most of the Sassanid Empire was overthrown by the Arabs over the course of 16 years in the 7th century. Although the administration of the state was rapidly Islamicized and subsumed under the Umayyad Caliphate, in the beginning "there was little serious pressure" exerted on newly subjected people to adopt Islam. Because of their sheer numbers, the conquered Zoroastrians had to be treated as "dhimmis" (despite doubts of the validity of this identification that persisted down the centuries), which made them eligible for protection. Islamic jurists took the stance that only Muslims could be perfectly moral, but "unbelievers might as well be left to their iniquities, so long as these did not vex their overlords." In the main, once the conquest was over and "local terms were agreed on", the Arab governors protected the local populations in exchange for tribute.