In popular accounts of the modern history of Islam, episodes of sectarian violence are invariably presented as the recrudescence of ancient, irrational hatreds that forever bedevil the region's inhabitants. Gaiser's fine monograph – which is emphatically not a book about modern sectarianism – is an intellectually responsible history of sectarian difference and of the emergence of a pentad of Muslim firaq, around which the book is largely organised: viz., the Khawārij and Ibāḍiyya, the Shīʿa, the Murjiʾa and Muʿtazila, and the Sunnīs (chapters 3–6, respectively). These parts of the book seek to complicate, while proving unable to fully transcend, the typologies and narratives of classical Muslim heresiography, a genre which first emerged in the Muslim Mashriq and drew upon longstanding Christian antecedents in the complex religious soup of late antiquity. Chapters 3–6 are bookended by chapters that theorize the notion of the “sect”, set the historical scene for the “core” of the book and reflect on the history of intra-Muslim relations (chapters 1, 2, and 8).

Identity is multifaceted and sectarian sentiments are not always salient; their “activation” takes place amid circumstances that are often highly localized (pp. 3, 180). Nor are sectarian identifications invariably “clear, exclusive or permanent” (p. 166). Sects are often misunderstood as having splintered from an original group, a tendentious misunderstanding that pervades Muslim heresiographical literature. In this vein, Gaiser opts (invoking Margaret Somers) to present Muslim sects as groups that coalesce around distinct narratives of salvation (the “narrative-identification” approach, p. 14): the narratives of the Shīʿa, e.g., privilege the imams of the Prophet's family and humans’ responses to them (p. 87). While the Quran itself emerged in a profoundly sectarian milieu, it was not until the death of the Prophet and ramifying debates over the succession to his rule that difference first became manifest. The Umayyads came to emphasize the importance of adhering to the community (luzūm al-jamāʿa, p. 148), and once the Abbasids had largely shed their Shīʿī tendencies, they too embraced this notion. The relative stability and prosperity of Abbasid rule, along with the relinquishing of caliphal pretensions to religious authority with the failure of the miḥna, were important contexts for the emergence of Sunnism, the last element of Gaiser's pentad to crystallize (p. 187). Gaiser also highlights the importance of ritual, mosque attendance and social organization to the articulation of early sectarian identity, particularly given their relative visibility (p. 104).

Irjāʾ is something of an exception among the trends analysed by Gaiser, as it is a sectarian story that lacks a founding narrative (p. 135), and Murjiʾī tendencies were largely adopted by emergent Sunnism. While the Muʿtazila share various (probably unhistorical) origin myths, by contrast, like the Murjiʾa they are more of a religio-philosophical grouping than a religio-political one, a reality made possible by the tabling of questions of communal leadership during the Abbasid period (p. 127). Among the Khawārij, one notices that while militant identities and narratives tended to solidify more quickly than quietist (not pacifist) ones, they proved less enduring (pp. 67–8). In this vein, the long subsequent development of the Ibāḍiyya leads Gaiser to distinguish them from other groups of Khawārij (a label they often rejected).

The simplicity of sectarian labels and typologies is belied by the complex history underlying intra-Muslim relations: the “extremist” (ghālī) Shīʿa seem to have interacted quite freely, in early Kufa, with their more moderate peers (including Jaʿfar al-Ṣādiq himself, p. 168), and Abū Ḥanīfa and al-Shāfiʿī both supported ostensibly Zaydī rebellions (p. 167). Even in contexts where sectarian identities were relatively salient and well-defined, they could still be de-prioritized in light of competing concerns, as in the case of ʿAlids joining Khārijī risings. Ultimately, there are simply too many variables to make it feasible to generalize about longue durée relations between sects, with the exception of the observation that widespread and long-term sectarian violence does not seem to have been the norm (p. 175). Even in the case of highly centralized confessional polities like the Ottomans, able to project their influence into the geographically remote and mountainous Levantine hinterland, heretical minorities such as the Nuṣayriyya were typically left to their own devices, as long as they paid their taxes (pp. 54, 171).

While it has not been possible to recount Gaiser's deft narration of the emergence of various Muslim sects and their inter-relations in any depth here, the concision of the book makes it ideal introductory reading for graduate and undergraduate students. Gaiser's creative synthesis of vast reams of existing scholarship also makes the work eminently useful for professional Islamicists. I do have a few minor reservations, however. The discussion of Sunnism is somewhat unsatisfactory, an inadequacy that reflects the state of the field more than it does Gaiser's scholarship (which has mostly concerned the Khawārij and Ibāḍiyya). Sunnism “cannot be said to have existed before the third/ninth century (at the earliest)” (p. 10), while al-Ashʿarī (d. 324/936) “wrote before something called ‘Sunnism’ could be said to have existed” (p. 18) and “the first recognizably Sunni figures were ḥadīth scholars from the late second/eighth century…” (p. 161). Gaiser suggests that the label Ahl al-sunna wa-l-jamāʿa postdates the activities of the ahl al-ḥadīth (p. 49) and seems to be unaware of the use of this term by early mutakallimūn including Ḍirār b. ʿAmr (d. c. 200/815). Given that Ḍirār's Kitāb al-Taḥrīsh functions largely as a polemic against the self-described Ahl al-sunna, it repays close attention, and one cannot but conclude that it is meaningful to speak of Sunnī identity in the period. To hold that the intense contestation of Sunnism in later centuries undermines this thesis, as Gaiser presumably would, is an example of the continuum fallacy: i.e. insofar as Sunnism is contested, it does not exist. This is evidently incorrect.

There are also a few factual errors: e.g. the claim that the Imāmiyya required the washing (as opposed to the wiping) of the feet in wuḍūʾ (p. 103). Additionally, al-Mughnī fī abwāb al-tawḥīd wa-l-ʿadl is mistranslated as “The Enricher of the Gates of God's Oneness and Justice” (p. 144), whereas mughnī suggests “sufficer” and abwāb has the sense of “aspects”.