In the immediate aftermath of the Second World War, almost all Pacific Islands and their people were firmly under colonial rule and therefore in no position to dictate the terms of regional organization. This was left to the founders of the first substantive regional organization, the South Pacific Commission, established in 1947 by Australia, New Zealand, the UK, the US, France and the Netherlands. The founders did, however, anticipate the involvement of Island people through the South Pacific Conference. This chapter focuses largely on the period of ‘colonial regionalism’ while also considering the wider context of post-war decolonization and the Cold War. Another important development in terms of defining the regional border with Southeast Asia came with the annexation of Netherlands New Guinea by Indonesia. Now known officially as the provinces of Papua and West Papua, they remain part of Indonesia, although self-determination issues are by no means settled.