Terrorist acts against the West have focused attention on fanatics who seek to overturn political and social order, even totally destroy society, in the name of an all-encompassing ideology. Comprehending the motivations of fanatical ideologues is critical to disrupting and preventing terrorism. The September 11 attacks as well as the London and Madrid bombings were planned and conducted by men who lived in the West. As it has repeatedly over the last century, knowledge that the perpetrators have lived among us and fear that confederates may yet be at large accentuates the security threat, while the hunt for home-grown traitors...