This paper analyses how political agency arises in islands caught in assemblages of competing international security projects. In this paper, we combine theoretical perspectives on geopolitical assemblages with examples of how island states come to exert effective sovereignty in international affairs through institutions which materialise out of non-sovereign entities. We also analyse how US-affiliated island jurisdictions in the Pacific navigate challenges like the US–China rivalry and the competing security doctrines of multiple states. As an example, we examined how jurisdictions with variegated forms of restricted sovereignty differed in their ability to manage the mobilities of US military personnel during the COVID pandemic. Through these discussions, we explore the potential for island states with limited self-determination to complete the project of decolonisation and to construct assemblages of governance which can protect the human and environmental security of inhabitants on islands while contesting the competing security initiatives of larger powers.