America's Transformation from Isolationism to Global Intervention: The Critical Years 1939–1941

This paper examines the gradual but decisive shift in American foreign policy from neutrality to full engagement in World War II during the period from 1939 to 1941. Through a comprehensive analysis of the modifications to the Neutrality Acts, the passage of the Lend-Lease Act, the formation of Anglo-American strategic cooperation, the escalation of U.S.-Japan tensions, and the outbreak of war following the Pearl Harbor attack, this study explores both the domestic and international factors driving the evolution of American diplomacy. Drawing upon Herbert Feis's seminal work The Road to Pearl Harbor and international relations theoretical frameworks, this paper demonstrates how external threats progressively narrowed the U.S. policy options, ultimately transforming isolationist America into the architect of the global anti-fascist coalition. The analysis reveals how economic pressures, diplomatic negotiations, and strategic miscalculations on both sides led inexorably toward conflict, fundamentally reshaping both the course of World War II and the post-war international order.