This book explores the origins of nationalism and the ideal of nation/state congruency since early-modern European thought, their transformation over time and endurance in contemporary political thought and IR theory. The author deploys a Lacanian-psychoanalytical reading of nationalism and the nation/state that goes beyond methodological nationalism and state-centrism critiques. He offers a genealogical inquiry into the emergence of the nation/state congruency ideal, thus exposing and problematising the practices that render nationalism and the ideal of the nation/state necessary. Offering a new way to read the ontology and epistemology of the nation/state, this work will be of interest to students and scholars of nations and nationalism, political thought, critical international relations and critical security studies.