American Economic History, Part III examines the transformation of the American economy and federal power from the 1920s through 2022. Beginning with Federal Reserve credit expansion, the Great Depression, and New Deal intervention, the course analyzes how monetary policy, regulation, taxation, spending, and uncertainty shaped investment, employment, and recovery. It then considers wartime planning, finance, procurement, and conscription alongside the lasting growth of the warfare and welfare states. The course follows the international monetary system from Bretton Woods through floating exchange rates, the dollar exchange standard, and its later erosion, connecting these regimes to inflation, recessions, financial crises, and changing Federal Reserve practices. Throughout, evidence on saving, production, employment, income, poverty, mobility, demographics, and asset prices provides a framework for evaluating postwar living standards and the relationship between market institutions, government intervention, and economic change.