The most effective international legal system in the world exists in Europe. It works much like a domestic system, where violations of the law are brought to court, legal decisions are respected, and the autonomous influence of law and legal rulings extends into the political process itself. The European legal system was not always so effective at influencing state behaviour and compelling compliance. Indeed the European Community's original legal system was intentionally designed to have very limited monitoring and enforcement capabilities. The European Court of Justice transformed the original system through bold and controversial legal decisions declaring the direct effect and supremacy of European law over national law. This book starts where traditional legal accounts leave off. Karen Alter explains why national courts took on a role enforcing European law against their governments, and why national governments accepted an institutional change that greatly compromised national sovereignty. She then shows how harnessing national courts to funnel private litigant challenges through to the ECJ and enforce European law supremacy contributed fundamentally to the emergence of an international rule of law in Europe, where national governments are held accountable to their European legal obligations, and where states actually avoid policies that might conflict with European law.