The invocation of fifth columns in the political arena -- whether contrived or based on real fears -- has recurred periodically throughout history and is experiencing an upsurge in our era of democratic erosion and geopolitical uncertainty. Fifth columns accusations can have baleful effects on governance and trust, as they call into question the loyalty and belonging of the targeted populations. They can cause human rights abuses, political repression, and even ethnic cleansing. Enemies Within is the first book to systematically investigate the roots and implications of the politics of fifth columns. In this volume, a multidisciplinary group of leading scholars address several related questions: When are actors likely to employ fifth-column claims and against whom? What accounts for changes in fifth-column framing over time? How do the claims and rhetoric of governments differ from those of societal groups? How do accusations against ethnically or ideologically defined groups differ? Finally, how do actors labeled as fifth columns respond? To answer these questions, the contributors apply a common theoretical framework and work within the tradition of qualitative social science to analyze cases from three continents, oftentimes challenging conventional wisdom. Enemies Within offers a unique perspective to better understand contemporary challenges including the rise of populism and authoritarianism, the return of chauvinistic nationalism, the weakening of democratic norms, and the persecution of ethnic or religious minorities and political dissidents.