This article investigates a hypothesis drawn from Martin Wight, that a society of states lacking a shared culture, as a result of expansion beyond its original base, will be unstable. This instability hypothesis has been influential in how the English School has presented the history of the expansion of European international society to a global scale. The article starts by offering two models of how a global international society could have come about since the late classical era: a multicultural encounter among several expanding civilizations (polycentric), or the takeover of the system by one centre (monocentric). Using these models as a backdrop, two accounts of the expansion story are developed. The Vanguardist account emphasizes the exceptionalism of European culture, posits a 500-year period of western domination, sees multiculturalism and the decline of western power as problematic, and tends to pessimism about the future of international society. The Syncretist account emphasizes the permanence of cross-cultural exchange, posits only a 200-year period of western dominance, sees culture and international society as evolving together, and is not pessimistic about the stability of international society. These two models and two accounts are then used to assess the possible future of international society. The article argues that culture is less of a problem for international society than Wight, and much of the English School, suppose. The evidence for the substantial success of syncretism is strong and provides considerable stability to most of the likely outcomes. The key problem is not culture, but socio-political structure. How can what North et al. call natural states and open access orders find shared practices and institutions that do not destabilize international society?