This review evaluates Toby Matthiesen’s extensive study of the historical development of the Sunnī-Shīʿī divide, highlighting its interdisciplinary approach and broad chronological scope. The book argues that the conflict between the two traditions has been shaped largely by political forces and evolving historical contexts. While acknowledging the work’s strengths—particularly its treatment of imperial and modern dynamics—the review critiques its methodological shortcomings, including insufficient critical engagement with sources and an apparent imbalance in presenting Sunnī and Shīʿī narratives. It also notes omissions and contested interpretations in key historical discussions. Overall, the review recognises the book as a significant but imperfect contribution to the study of sectarian history in Islam.