Towards the end of this entertaining, informative volume, its author remarks that, in the wake of World War II, there was “much uncertainty and division in the Muslim Middle East,” and that this gave rise to “discussions about the future and the revival and development of the area.” At the time, however, few, if any, of the region’s leaders gave any thought to the restoration of the Muslim caliphate. Most were nationalists and modernizers, and many were outright socialists.

Egyptian President Abdel Nasser’s nationalization of the Suez Canal in 1956, and the subsequent humiliation of British and French attempts to take it back, led to the forging of close links between Egypt and the Soviet Union. For most politically active people in the Arab Middle East, communism rather than the revival of the ancient caliphate pointed the way to the future. When I first started travelling in the area in 1964 it was generally assumed this was where the future lay. Most people would have accepted that they were Muslims as well as socialists and nationalists, but the mosques were for old men, relics of a vanished world, and the caliphate as distant and as irrelevant as the Holy Roman Empire in post-war western Europe.

In the interim, Hugh Kennedy ruefully acknowledges in Caliphate: The History of an Idea, things have changed—a lot. Communism ended up in the ash heap of history, and, although he does not say so, Arab nationalism is today—to an ever increasing degree—the faith of old men, a relic of a rapidly vanishing world.

This was already clear 20 years after Kennedy first arrived in the Middle East, when I set off for Istanbul to spend two years in Turkey, Greece, and Cyprus as a fellow of the Institute of Current World Affairs. In Turkey, the disciples of Kemal Atatürk, the country’s modern founder, were still firmly ensconced in power. But in Anatolia and in the poorer districts of Istanbul, Ankara, and Izmir, where the newly arrived country folk resided, Islam was enjoying a resurgence, and people dreamed that the caliphate might be restored. For the most part, I hobnobbed with the same sorts of people that Kennedy met in the Arab world. My acquaintances harbored nothing but contempt for the Islamists from the market towns and for the village folk who supported them; but, even then, they were afraid.

* * *

A professor of Arabic at the school of Oriental and African Studies (SOAS) at the University of London, Kennedy knows a great deal about the contemporary Middle East, and has written a slew of books on early Islam and its development in its first millennium in the Middle East and in Spain and Portugal. In his new book, he draws on what he knows well to tell us about the origins of the caliphate: about its articulation under the Umayyads of Damascus and the Abbasids of Baghdad, and about its subsequent history in Fatimid Egypt, in Umayyad and Almohad Iberia, and under the Mamluks of Cairo and the Ottomans of Constantinople. This he does to a considerable degree by quoting extensively from contemporary or near-contemporary reports. The upshot is that he allows us to see what it is that inspires nostalgia for the caliphate within the Muslim Brotherhood, the adherents of the Hizb al-Tahrir and of al-Qaeda, and the supporters of ISIS.

Kennedy’s book is not the first on the subject. In 1924—the year in which Atatürk and the Grand National Assembly in Turkey abolished the caliphate—Sir Thomas W. Arnold, perhaps the most distinguished of Kennedy’s predecessors at SOAS, published an accessible monograph on the subject. Kennedy profits from Arnold’s pioneering work, from the extensive scholarship that has been done in the interim, and from the documents that have more recently become available. He is especially good on the four orthodox caliphs, on the Umayyad dynasty, and on the Abassids who held the office and presided over much of the Middle East (if they did not always rule it) for nearly 800 years.

* * *

In Arabic, the word khalifa has two meanings. It can be used to refer to a deputy, and it appears to have been employed by Muhammad himself to refer to those left in charge at Medina when he was abroad. It can also be used to refer to a successor. The death of Muhammad was in one respect similar to the death of Jesus Christ. It left his followers nonplussed, and the movement he had founded could easily have melted away. That it did not do so was the work of his companions, who did for Islam what Peter, Paul, and the apostles did for Christianity: they took the following of a charismatic leader and turned it into a revolutionary movement intent on converting the entire human race.

When Muhammad died, his companions moved quickly to form a shura or council and install Abu Bakr as the caliph. It was during his brief reign (632-34) and the longer reign of his immediate successor, Umar (634-44), that the Arabian Muslims surged forth from the Hijaz and conquered Iraq, Syria, and Egypt. And it was during the reign of their successor, Uthman (644-56), another of Muhammad’s companions, that they consolidated control. It was in these years, moreover, that the Koran was compiled and edited; that the terms governing relations between the Muslims and the Jews, Christians, and Zoroastrians they ruled were articulated; that the conquerors were settled in garrison towns in Syria, Iraq, and Egypt; that the bureaucracy and the system of taxation were established; and that the minting of the coins necessary for the support of Muslim rule began.

Historians have long known that the early caliphs served as the leaders of the umma, or united community of the world’s Muslims. But whether they exercised authority as deputies of Allah or as successors of Muhammad has long been a subject of scholarly debate. They did not claim to be prophets, for Muhammad had made it clear that he was the last in that long line. But they were anything but figureheads, and, as Kennedy points out, the surviving evidence from the period (most of it poetry) suggests that they were thought to be stand-ins for Allah and not (like the Abbasids in and after the middle of the 9th century) mere successors.

* * *

In one very important regard, the religious movement founded by Muhammad’s companions was very different from early Christianity. To ask whether the early caliphs were political or spiritual leaders is to draw a distinction alien to their understanding. Islam is less a religion of faith than a religion of Holy Law and of observance. Maintaining Muslim armies, conducting the jihad, protecting the hajj, taxing the dhimmis, articulating the sharia and enforcing it—acts that we might take to be military and political—were for these men first and foremost works of piety. If the Islamic revival of the last few decades has been politically disruptive, if the Muslim minorities in the liberal democracies of Europe are ripe for revolutionary incitement, it is because classical Islam brooks no distinction between what we think of as religion and political rule.

In one particular, what the early caliphs did was ad hoc. Muhammad seems not to have anticipated his own demise, and he did not indicate who was to lead his followers after his death. Nor did any of the early caliphs establish a mechanism for selection. Sometimes, a shura was summoned. Generally, the existing caliph designated a successor. Sometimes he named not only his successor but his successor’s successor; and there were times when he died without designating any sort of successor at all. When quarrels arose—over questions of doctrine and political practice, or over misconduct (real or imagined) on the part of a caliph—assassinations took place. Civil wars were common, and eligibility for the office was disputed.

Did the caliph have to come from Muhammad’s family? His son-in-law Ali did, but the three men who preceded him as caliph did not. Did he have to come from Muhammad’s clan, the Quraysh? The first four caliphs did, as did the Umayyads in Damascus and Cordoba, the Abbasids in Baghdad and Cairo, and the Fatimids in Egypt (or so they claimed). But there were those who argued that all Muslims should be eligible and that the most pious of the lot should be chosen. Others thought that military prowess (shawkat) was not only necessary but, when supplemented by a semblance of piety, sufficient. It was even disputed by those who insisted that the caliph be drawn from the family of the prophet whether the descendants of his uncle and guardian—the Abbasids—actually fit the bill. And some insisted that there needn’t be one caliph, but could be many—one in Baghdad, another in Egypt, and a third in Cordoba.

* * *

These disputes gave rise to sectarian divisions—the distinction between Sunni Islam and the various and opposed Shiite sects—and, even within a particular sect, the absence of clarity about the grounds for legitimacy frequently gave rise to war and instability. Among the Sunnis, there were Abbasid caliphs, lodged in Baghdad and, later, Cairo, from 750 to 1517. But they were generally not without rivals and, after 945, they were little more than pawns, supported in comfort and, sometimes, splendor on condition that they legitimized the Buyid, Seljuq, or Mamluk thugs who actually ran the show.

Kennedy does a splendid job in surveying the vagaries of Muslim practice in all these regards, and he touches from time to time on theoretical and legal works dedicated to explicating the foundations, rights, and duties of the office. Here, though, his account has its defects. He does allude to the flourishing of philosophy in 9th- and 10th-century Baghdad, and he touches on its brief efflorescence in Cordoba in the time of Islamic philosophers Ibn Tufayl and Averroës. He mentions neither Alfarabi nor Avicenna, however, and he takes it for granted that the falasifa were pious Muslims, illustrations of “fides quaerens intellectum” (faith seeking understanding). Had he attended to their treatment of religion under the rubric of political science and their refusal to subordinate political prudence to the dictates of religious faith, he might not have given such short shrift to the charges of impiety lodged by al-Ghazali in The Incoherence of the Philosophers.

Moreover, when Kennedy discusses the 14th-century historian and political scientist Ibn Khaldun, he fails to address his discussion of whether the caliphate—in his day, toothless—should be abolished. (No, he answered.) Although Kennedy mentions the abolition of the caliphate in 1924, he doesn’t tell us why Atatürk did so. He says very little about the role assigned (or denied) the caliphate by Turkish nationalist thinkers in the 19th and early 20th centuries, and he does not do full justice to the arguments advanced by those in the Muslim Brotherhood, the Hizb al-Tahrir, al-Qaeda, and ISIS who favor its restoration. Caliphate: The History of an Idea provides us with a context within which to understand the debates on this subject conducted within the last century-and-a-half within Islam, but Hugh Kennedy does not bring his learning to bear on a systematic analysis of these debates, and he does not explain to us what is going on today, why and how it is important, and in what manner the disputes are likely to be resolved. That important task still needs to be undertaken.