Claims about the activities of fifth columns are experiencing an upsurge in our era of democratic erosion and geopolitical uncertainty. This pathbreaking multidisciplinary volume brings together leading scholars to break new ground in the study of fifth columns and the politics that surround them. It uses an original theoretical framework within the tradition of qualitative social science and analyzes cases from three continents. Enemies Within offers aunique 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.