Muhammad’s immediate successors, called caliphs (pronounced KAYliff), inherited an expanding but loose-knit social fabric. The Prophet had united the Bedouin tribes under the banner of Islam, but tribal loyalties cooled quickly when the leader died. When Muslim elders in Medina chose Muhammad’s father-in-law Abu Bakr as the first caliph, the initial challenge was to regather the tribes already reverting to their pre-Islamic ways. Umar (634-644), the second caliph, then mobilized tribal forces to move northward into Syria and Mesopotamia (Iraq), westward into Egypt, and eastward into Persia. Next, Umar instituted important policies in the conquered lands, allowing the subjected peoples to retain their religion and law, and levied taxes often lower than what had been paid previously to Byzantium and Persia. Muslim armies remained apart in garrisons that eventually became cities in their own right. Umar’s successors, Uthman (644-656) and Ali (656-661), compassed the downfall of the last Sasanian emperor, but had to deal with disastrous internal strife as well.