This paper seeks to reconstruct the worldview informing Iberian overseas expansion during the long sixteenth century, arguing that this worldview was more indebted to Renaissance cosmology than to a recognisably modern scientific worldview. The paper describes how this cosmology provided the intellectual resources necessary to justify overseas expansion to those who doubted its viability and legitimacy, and how the same cosmological beliefs were invoked to make sense of the New World and the people found there, if only to facilitate and justify the subjection of the latter to European rule. This story constitutes an important yet often neglected part of the prehistory of modern international thought insofar as it exposes its Iberian origins and Renaissance foundations and the role played by pre-modern ideas in the making of a modern international system.