This chapter examines the escalation of U.S.–Japan tensions that culminated in America’s entry into World War II (WWII) and assesses the profound consequences for Japan’s political and later constitutional identity. Japan’s imperial expansion in Asia generated successive diplomatic crises and military confrontations, ultimately leading to the atomic bombings of Hiroshima and Nagasaki. Emperor Shōwa’s decision to surrender brought an end to wartime and inaugurated the Allied occupation, during which Japan underwent sweeping demilitarization, a fundamental redefinition of the emperor’s role, and adoption of a new constitution. By linking prewar diplomacy, wartime escalation, and postwar reforms, this chapter demonstrates how U.S. strategic choices and occupation policies reshaped Japan’s system of governance and international orientation, consolidating its transformation from an imperial power into a pacifist state.