By the time of the Abbasid revolution in the middle of the 8th century, Muslim armies had come against a combination of natural barriers and powerful states that impeded any further military progress. The wars produced diminishing returns in personal gains and fighters increasingly left the army for civilian occupations. The priorities of the rulers also shifted from conquest of new lands to administration of the acquired empire. Although the Abbasid era witnessed some new territorial gains, such as the conquests of Sicily and Crete, the period of rapid centralized expansion would now give way to an era when further spread of Islam would be slow and accomplished through the efforts of local dynasties, missionaries, and traders.