The project of restoring the Islamic caliphate has attracted considerable attention, and caused both hope and anxiety, since the Islamic State (IS) proclaimed a caliphate in 2014. The idea of a caliphate has a powerful attraction for many millions of the world’s Muslims, including those who emphatically disapprove of IS, and is sometimes viewed as a remedy for the marginalization and humiliation that the Muslim world has endured at the hands of the West. Thus, although the military defeat of IS seems likely at this point, a widespread desire for a restored caliphate will undoubtedly persist.On the other hand, prominent Western political leaders, diplomats and scholars have expressed severe forebodings about such a restoration. Among other causes for such concern is the fear that a restored caliphate would challenge, and might eventually overturn, the Westphalian system of state sovereignty, which is the legal basis of the current world order. From this perspective, a restored caliphate could pose a revolutionary challenge to a state-centric system of international law. Indeed, it might pose a more substantial threat to the Westphalian system than those that the Soviet Union or the Islamic Republic of Iran presented in their early phases.This paper explores whether the project of restoring a caliphate, if successful, would indeed have such a destabilizing effect. First, it considers the origins and self-understanding of the IS caliphate, examines IS’ historiographic narrative of Islamic expansion, greatness, decline and eventual renewal, and identifies structural reasons for IS’ probable failure. Then it examines the possibility of what has been called a “Great Power” caliphate, i.e., a restored caliphate in a major Muslim state, such as Turkey, Egypt, Pakistan or Indonesia. Here too, it is argued, the Westphalian Order would probably prove to be resilient enough to withstand a challenge.