Under the UN Convention on Biological Diversity (CBD) Aichi biodiversity targets, nations have committed to conserving 10% of the oceans within their territories by 2020. Over the past decade, this goal has driven the establishment of many large marine protected areas (MPAs), several of which surround overseas island territories with current or historical military involvement, ranging from World War 2 battle sites to testing areas for the “ABCs” of atomic, biological, and chemical weapons during the cold war. For countries with significant overseas territories, such as the USA, France, and the UK, these remote possessions provide an opportunity to achieve biodiversity conservation objectives over large spatial scales. They also provide a strategic footprint for regional maritime spheres of influence, as well as possible future energy and mineral resources. Building on insights from terrestrial “militarized” protected areas, and drawing on archival and contemporary sources, this paper examines the multiple motivations behind designating very large MPAs in overseas territories, from protecting biodiversity to more long-term geopolitical, security, and resource-oriented motivations.