The institution of leadership of the Muslim umma (community).
After the death of the Prophet Muhammad in 632, the Muslim community found itself faced with a dilemma: the Qur’ân provided no clear indication of who was intended to lead the umma after the Prophet, nor in exactly what capacity. Leadership of the community soon passed to individuals who became known as caliphs; the English term derives from Arabic khalifa (successor). The caliphs were initially, de facto, both military commanders (as leaders of raids were in the pre-Islamic era) and religious leaders (in their capacity as successors to the Prophet, although they were not prophets themselves).
By the time of the crusades the relationship between the caliph and the actual mechanisms of rule had become less stringently defined as a result of ninth-century tensions between the caliphs and the religious elite as well as later developments in theories of the caliphate. From the ninth century on, domination of the caliphs by their military subordinates also restricted their power. The situation came to a head in 946, when the caliph al-Mustakfi (944-946) was forced to accept a member of the Shi‘ite Bûyid family as his deputy, becoming little more than a figurehead for the new ruler of the Muslim world. While the Bûyids were displaced by the Sunni Saljûq Turks in 1055, the caliphs remained for the most part the religious and the secular leaders of the Muslim world in name only. Nevertheless, during the crusading period some caliphs, most notably al-Muqtaf (1136-1160) and al-Nasir (1180-1225), managed to resist their theoretical subordinates and assert their own authority.
The first four caliphs were chosen by the general agreement of the umma, but after the death of the fourth caliph, ‘Alī ibn Abi Tālīb (661), the caliphate passed to the Umayyad family, who instituted a principle of dynastic succession, something that raised some opposition within the umma. By the eighth century, the Umayyads had become embroiled in a number of difficulties and had made several major enemies. Finally, in 750, they were replaced by another prominent Arab family, the ‘Abbāsids. ‘Abbāsid caliphs reigned (though as indicated, few actually ruled) at Baghdad until 1258 and at Cairo from 1262 (under Mamlûk tutelage) until the fall of the Mamlûk sultanate to the Ottomans in 1517.
At times there were rival caliphates, each of them claiming to be the legitimate one. Partisans of the family of ‘Ali had opposed both the Umayyads and the ‘Abbāsids; indeed, in North Africa theShī‘ite Fātimids founded a caliphate at Kairouan in 909, took Egypt sixty years later, and ruled there until 1171. Rival caliphates also existed in Spain, where the Umayyad emir, ‘Abd al-RahmānIII (d. 961), founded a caliphate that lasted from 928 until 1031, and in North Africa, where the Almohad ‘Abd al-Mu’min (d. 1163) and his descendants claimed the title until 1269. The title was also claimed by the Hafsids of Tunisia and eastern Algeria (1228-1574) and the Marinids of Morocco (1196-1464).