Attempts to analyze and understand how European law developed from a set of international treaties in the 1950s to a constitutional, proto-federal legal order, accompanied by a constitutional legal discourse today, has been a key concern in European studies in the last three decades. Legal scholars, political scientists, and sociologists have explored this from their specific disciplinary viewpoints and have produced a rich literature of sophisticated theoretical as well as empirical studies. Since the mid-2000s, historians have also finally—after years of negligence—taken an interest in European law and produced a new body of archive-based studies of the history of European law from 1950 to 1993. Based on primary sources drawn from private, national, and European archives, historians have contributed with much new empirical information and managed to uncover the social, political, and legal forces that have shaped European law in a qualitatively new way. The central argument is that the constitutionalization of European law was part of the broader battle over the political and institutional soul of the European construction. Even though the ECJ successfully constructed a European legal order that resembled and worked as a proto-federal constitution, the project ultimately suffered a defeat in not being able to codify this achievement in the Maastricht Treaty as part of a broader step toward a federal Europe.