This introductory chapter presents the reader to the book’s main puzzle and analytical contributions. It challenges the theoretical and normative division between police and armed forces, as well as internal and international security, that tend to frame the domestic deployment of the armed forces as a rupture with the normal uses of the military, arguing that the main question to be asked is how the line between acceptable and unacceptable uses of the instruments of state lethality—i.e. the armed forces—is drawn. The chapter positions the book as a theoretical intervention into debates about violence, legitimacy, and the evolving involvement of the armed forces in domestic security, grounded on the empirical analysis of the Brazilian case.