Cartels today are illegal and illegitimate across the globe. Yet this is only a recent post-World War II phenomenon, especially in Europe. So how did cartels become bad? During the 1920s-1930s, over 3,000 monopolistic business agreements governed the world economy. Meanwhile, European governments, politicians, economists and industrialists championed cartels as a near-panacea for stabilizing the continent’s political and economic order established in the wake of World War I. Overproduction, violent price fluctuations, mass-unemployment and rising protectionism shattered most contemporaries’ faith in laissez-faire capitalism. “Industrial self-government", or producers’ regulation of prices and output via cartels, offered an attractive alternative to the rise of statist planning. Many even promoted cartels as the path to peace and a federal “United States of Europe.” But this consensus, which my dissertation conceptualizes as “cartel capitalism,” crumbled after 1945. Most governments outlawed cartels, while European integration was built on supranational cartel control. My project investigates the causes of this great reversal and maps the rise and fall of cartel capitalism onto broader transformations in projects to govern and unify the European economy, notably in the shift from business-led planning in the 1920s to state-led economic management in the 1950s. I situate the origins of Europe’s anti-cartel turn not in the Americanization of European economies, as most scholars have, but in Europe’s own reckoning with WW2. Cartels became delegitimized through their association with Nazism, collaboration, treason, and war crimes. I then demonstrate how post-war reconstruction projects in Britain, France and West-Germany were predicated on state conquest of private monopoly power. Since the 1980s-90s, the European Union has usurped America's traditional antitrust leadership. To uncover the foundations of the world's most robust competition regime and revitalize antitrust for our present, we must return to the lesson of the 1940s: that monopolistic big business threatens sovereignty, democracy and peace.