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.

Advances in long-range precision strike missiles, such as cruise missiles, ballistic missiles and hypersonic weapons, threaten the survivability of surface warships. In response, states are deploying submarines in the water column and expanding military activities on the seabed. Concepts such as “upward falling payloads” and networked “hydra” seabed installations envision prepositioned sensors and weapons emplaced on the continental shelf along an adversary’s coast. This chapter explores the legality of military operations on the continental shelf of a coastal state. The coastal state has sovereign rights and jurisdiction over the living and nonliving resources of the seabed and subsoil in accordance with the UN Convention on the Law of the Sea. Foreign military activities are generally permissible on the continental shelf of a coastal state so long as they observe due regard for the resource rights of the coastal state. Coastal states enjoy the exclusive right to construct artificial islands on their continental shelf but they lack competence to regulate military seabed installations and structures on the continental shelf.