This book investigates politics of denaturalisation as a system of thought that influences seminal cultural political values, such as community, nationality, citizenship, selfhood and otherness. The context of the analysis is the politics of citizenship and nationality in France. Combining research insights from history, legal studies, security studies, and border studies, the book demonstrates that the language of denaturalisation shapes national identity as a form of formal legal attachment but also, and more counter-intuitively, as a mode of emotional belonging. As such, denaturalisation operates as an instrumental frame to maintain and secure the national community. Going back to eighteenth-century France and to both World Wars, periods during which governments deployed denaturalisation as a technology against threatening subjects, the analysis exposes how the language of denaturalisation interweaves concerns about immigration and national security. It is this historical backdrop that helps understand the political impact of denaturalisation in contemporary counterterrorism politics, and what is at stake when borders and identities become affective technologies.