The international system is becoming increasingly legalized, with legal arguments and legal advisors playing an increasingly important part in the state policymaking process. Presenting a practice-oriented theory of compliance with international law, this book shows how international law affects the behavior of increasingly lawyerized states in an ever more legalized world. By highlighting the legalization of international legitimation and the lawyerization of policymaking as the new engines of compliance, the book’s analytical framework rethinks the relationship between state behavior and international law, and provides an empirical focus on security through the study of NATO’s military intervention in Yugoslavia in 1999 and the changes in the US detention and interrogation programs in the "War on Terror." Relying on primary sources, the author demonstrates the effect of lawyerized decision making on international law compliance, reconstructing the strategies of (de-)legitimation used to show that international law is the hegemonic frame of reference in interstate debates. This book will be of interest to scholars of international relations, government studies, foreign service studies and lawyers employed in government work.