From their establishment in the nineteenth century, international (intergovernmental) organizations (IOs) have been intimately linked to both international and domestic aspects of state making. This chapter examines non-European IO membership in the nineteenth century and argues that joining an IO could strengthen a state’s claim to statehood in two ways. First, IOs provided an arena of international politics where non-European states could participate on the basis of formal sovereign equality with the European great powers. Second, joining IOs and implementing their agreements on postal services, telegraphs, intellectual property, and other new technologies and government services, further offered a way for non-European states to prove that they were doing what ‘modern’ states were supposed to do. IO membership thus offered the possibility for non-European states both to gain international recognition as sovereign equals, and a means for them to display their progress in extending their domestic jurisdiction within their territories. The chapter problematizes the Westphalian unilinear view of state making, as well as the English school expansion thesis, by examining the agency of non-European entities and how their decisions to join IOs both strengthened individual states’ claims to statehood, and contributed to changes in international society.