The modern international system is commonly argued to have originated within Western Europe and spread globally during centuries of colonialism. This article argues, instead, that the character of the modern system of territorially sovereign states resulted from a complex interaction between European colonizing polities and events, actors, and spaces in other parts of the globe. In particular, through a process of colonial reflection, many of the foundational ideas and practices of modern statehood were formed in the interactions of Europeans with the unknown, supposedly empty, spaces of the New World in the 16th and 17th centuries. These novel practices were applied only later to politics among states in Europe. Most important among these developments is the ideal of territorial exclusivity as the sole basis for state sovereignty. This analysis also has implications for the study of contemporary international systemic change.