A central component of the Japanese government’s foreign policy after the First World War was its desire to control and secure access to strategic minerals necessary for munitions production. Two-thirds of the world supply of these strategic minerals—including metals like copper, lead, molybdenum, nickel, tungsten and zinc—was primarily under the commercial control of American and British interests. American geologists and foreign policy experts assumed that Anglo-American sanctions combined with the lack of mineral deposits in East Asia would prevent Japanese attempts at economic autarky. Yet these experts failed to appreciate the importance of the Korean peninsula. Korean minerals—in particular tungsten—enabled the Japanese war economy to partially withstand the economic strangulation of the Allied blockade and bombings in the last years of the war. After the war, Korean scholars documented, in great detail, the Japanese exploitation of these resources, yet this scholarship does not show how these Korean resources fit into the larger resource base of the so-called “Co-Prosperity Sphere.” This article examines the origins, development, and consequences of Japan’s exploitation of Korean mineral resources from 1910 to 1945 through a case study of tungsten and uses two distinct sets of sources: first, reports and studies produced by American geologists from the 1920s to the postwar period and, second, Japanese mining reports and statistics produced in Korea and Japan during the colonial period.