This book examines federalism and functionalism – two fundamental, yet largely forgotten, theories of international integration. Following the recent outbreak of the war in Ukraine, policy practitioners and scholars have been in search of a deeper understanding of the likely causes of the conflict and its consequences for the European security architecture. Various theories have been deployed to this end, but international and European integration theory remains conspicuously absent. The author shows how the core tenets of integration theories developed after World War I, particularly how they viewed territoriality and geopolitical boundaries, remain as relevant today as they were almost 100 years ago.