This book addresses one of the most profound transformations in international governance: the proliferation of regime complexity. Regime complexes can be found wherever state interests clash. Thus, even in one of the most constitutionalized of institutional environments, the European Union (EU), regime complexity features prominently - especially in European defence cooperation, where states have created competing institutions overlapping in their mandates to organize armaments cooperation or defence planning. The tense relationship between the institutions of the EU's Common...