Shows how religious fanaticism became a tool used to police subversive and targeted religions at home and abroad In 1822, Thomas Jefferson wrote that the "atmosphere of our country is unquestionably charged with a threatening cloud of fanaticism." Indeed, during the nineteenth century the United States was full of radical theologies, messiahs, utopian dreams, passionate exhortations, and sacred violence. This book seeks to uncover the history, rationales, and effects of understandings of religious fanaticism, and how the term was wielded to describe and denigrate a diverse array of religious groups in the United States. American Fanatics traces the development and popularization of religious fanaticism - a precursor to today's categories of religious terrorism, radicalism, and extremism - and explores the violence hidden in its usage. From the Second Great Awakening in the early 1800s to the US occupation of the Philippines in the early 1990s, the book follows the rise of the concept through distinct conflicts over evangelical revivals, abolition, literature, psychiatry, and colonial anthropology. It charts how the term "fanatic" started out as a marker for excessive religious practices, but evolved into a religio-racial category that framed resistance to power as overly emotional, delusional, and inherently violent. American Fanatics illuminates how from the colonial period to the nineteenth century, Americans transformed "fanaticism" from a term of Christian theology into one of religio-racial security, wielding it as a tool of domestic and imperial governance.