In the recent flood of accounts of radical Islam, this one stands out. Maher’s compelling exploration of Salafi jihadism achieves a level of clarity that perhaps could be produced only by someone who, like Maher, once adhered to that strain of thought. The book is exceptional also in its focus on theology: although Maher is a specialist in jihadist radicalization, he dwells little on jihadists’ motivations, paying much more attention to their beliefs. Salafi jihadism rests on five doctrinal building blocks that together create a coherent and consistent ideology: jihad (holy war), tawhid (the oneness of God), hakimiyya (true Islamic government), al-wala wal-bara (loyalty to divine truth and disavowal of untruth and polytheism), and takfir (the naming of disbelievers). (There are some partially irreconcilable tenets, however, when it comes to the killing of innocents.) This extremist creed reflects core Islamic beliefs. But the contemporary appeal and spread of Salafi jihadism have been most profoundly shaped by the civil war in Algeria in the 1990s, the 2003 U.S.-led invasion of Iraq, and the ongoing turmoil in Afghanistan. The unending conflict in Syria will lead to the further refinement and growth of this form of radicalism, and not to its demise.