Few attributes are more important to the European Union than that of Rechtsstaat. The connotations of the term are both internal and external: it can refer either to the supremacy of law within its own borders, or to its respect for international law beyond them. Historically, the “law of nations”, as it was originally called, predates the jurisprudence of the Union, a relatively recent creation, by many centuries. At its centre lay a concern with the regulation of armed conflicts between peoples and States: relationships of power traditionally settled by war. What is the bearing of these on a contemporary political system priding itself on its commitment to peace? This series of lectures will look, in turn, at the origins and evolution of the law of nations in Europe; at the origins and nature of law within a Union that defines itself as neither national nor international; at the kinds and results of the ensuing statecraft and democratic will-formation; and in conclusion, at the historical juncture of the Union at the time of war in Ukraine.