In forensic and compelling detail, Jytte Klausen traces how Islamist revolutionaries exiled in Europe and North America in the 1990s helped create and control one of the world's most impactful terrorist movements-and how, after the near-obliteration of the organization during the aftermath of the 9/11 attacks, they helped build it again. She shows how the diffusion of Islamist terrorism to Europe and North America has been driven, not by local grievances of Western Muslims, but by the strategic priorities of the international Salafi-jihadist revolutionary movement. The jihadists-Al Qaeda and the Islamic State, and their many affiliates and associates-also proved to be amazingly resilient. Again and again, the movement recovered from major setbacks. Appealing to disaffected Muslims of immigrant origin and alienated converts to Islam, jihadist groups continue to recruit new adherents in Europe and North America, street-side in neighborhoods, in jails, and online through increasingly clandestine platforms. Taking a comparative and historical approach, deploying cutting-edge analytical tools, and drawing on her unparalleled database of up to 6,500 Western jihadist extremists and their networks, Klausen has produced the most comprehensive account yet of the origins of Western jihadism and its role in the global movement.