Arab tribes had come together for a common cause in the pre-Islamic period, such as a war against another tribe or recognition of the strength of a chieftain. But once that cause had been accomplished or that chieftain had died, the confederacy typically disbanded, its purpose fulfilled. In the wake of Muhammad’s death, at least some Arab tribes likewise believed the community’s purpose had been completed. His accomplishment in bringing people together under the banner of Islam was not one the surviving leaders of the community intended to be temporary, however. There were urgent questions about the leadership of the community, and immediate disagreements about it as well. In many tribal- and clan-based societies like that of the Arabs, leadership was not hereditary, meaning it did not immediately pass to the heir upon the death of the leader. Thus, as Muhammad was dying, two primary claimants for leadership emerged: his son-in-law Ali ibn Abi Talib, and a friend and confidant of Muhammad’s named Abu Bakr. Ali, related to the Prophet by blood and marriage, was comparatively young in a society that saw leadership in its elders, although he claimed to have been chosen by Muhammad as his heir and successor. Abu Bakr, in contrast, was one of the elders of the community, well respected and popularly chosen. Both had been among the first to convert to Islam. Members of the community had concerns about their own standing. Some were the earliest converts who had joined Muhammad when he was still in Mecca, some had welcomed his community in Medina when they needed shelter, and still others had not converted until shortly before Muhammad’s death. In the end, Abu Bakr was chosen to be the first successor to Muhammad, the caliph or religious and political leader of the Muslim community. This was accomplished through popular acclamation by tribal leaders, who ultimately won out over those who favored the lineage of Ali. Islam had weathered this first hurdle, although the question of leadership had longer-term implications for the unity of the ummah. Ali, believed by some to have been chosen by Muhammad as his heir, was likely aggrieved at the decision, although he accepted it. Other stakeholders may also have felt slighted, including a number of Muhammad’s wives, several of whom were shunned despite their close relationship as members of the family of the Prophet. And while this new role of caliph would provide leadership to the young community at a critical juncture, there seems to have been near immediate recognition that things without Muhammad would be different, not least of which because the caliph was not assuming the mantle of another prophet capable of communicating directly with God as Muhammad had. Tensions arose after Muhammad’s death not just over leadership and inheritance, but also over whether the alliance was ever intended to last beyond its founder. Some Arab tribes left to return to their homes, while others may have believed they could discard their commitment to the worship of the one God and membership in this confederation. From the perspective of the Muslims, however, this was apostasy, and a conflict known as the Ridda Wars then began in an attempt to force these Arab tribes to continue to honor their agreements with the Muslims. The Ridda Wars also appear to have been expansionist, bringing into the fold, whether by treaty or force, Arab tribes that had never been aligned with Muhammad’s community during his lifetime. This effort was the first step of a wider movement called the Arab-Islamic or Arab-Muslim conquests, and by 633 the entirety of Arabia had been brought under the control of this first Islamic state. Abu Bakr did not live long after Muhammad, and the conquest movement did not stop with his leadership, nor with uniting just the Arab tribes under the banner of Islam. The new state’s expansionist desire seems to have existed from the outset, and the Arab-Muslim armies turned their attention northward to the old empires of Sasanian Persia and Byzantium. They were likely inspired by the richness of these lands, where they knew resources were more plentiful and luxury trade goods regularly traveled. But there were other factors, too. The Arab-Muslims may have felt emboldened by their successes in Arabia, seeing them as recognition of God’s favor and of their destiny to rule the world. Religious belief and zeal are difficult for historians to quantify, but we have seen throughout history that nomadic and seminomadic societies must forcefully seek the resources they need to survive while defending themselves against threats that sedentary societies face less often. The hardiness and capability of the Arab-Muslims as a fighting force during this period was also a factor. The weakness of the empires to the north would have been seen as a clear opportunity for the raiders who had long supported themselves by harrying the frontiers. And there was the timing: Muhammad and his successors were creating and expanding the new Muslim community in the 620s and 630s, as the war between the Byzantines and the Sasanian Persians was entering its last stages and leaving both empires weakened at a critical juncture.