In Clerics in Contention, Alexandre Pelletier investigates why Islamist mobilization takes root in some regions of Java but not in others. Drawing on a dataset of thousands of Islamic schools and clerics, more than a hundred interviews, and archival research, he shows that West Java's religious field is uniquely fragmented and competitive. In that environment, clerics face constant pressure to compete for followers through evermore provocative mobilization. Islamist movements thus flourish where Islamic authority is fragmented and competitive.Pelletier traces these patterns back to Dutch rule, showing how colonial land regimes and administrative structures produced enduring divergences in religious authority. Where communal land and village institutions existed, as in East Java, clerics consolidated lasting authority; where private property and extractive colonial control prevailed, as in West Java, religious authority fractured. These legacies forged enduring local political cleavages and left West Java more vulnerable to Islamist mobilization. Clerics in Contention rethinks the history of Islam in Indonesia, demonstrating that moderation and radicalism arise not from belief alone, but from the institutional world that anchors religious authority.