This chapter explores how the strategic value of bird islands increased in the interwar period, even as their economic value dwindled. The Marcus Island Incident helped spark Japanese interest in offshore guano mining for use as phosphate fertilizer, and Japanese-managed mining operations began to pop up on islands throughout the East and South China seas. They were only intermittently profitable, and were abandoned during economic downturns. But they triggered diplomatic disputes first with China (over the Pratas and Paracel groups) and then with France (over the Spratlys). Over time military planners began to conceive of the islands as potential airstrips or submarine refuelling stations. Japanese companies, often competing with each other for rights to the islands, exploited these visions by portraying themselves as useful adjuncts in the defence of Japan’s ‘maritime lifeline’. By the late 1930s the Japanese Navy was directly bankrolling civilian enterprises as cover for military operations.