Medicines that are vital for the saving and preserving of life in conditions of public health emergency or endemic serious disease are known as essential medicines. In many developing world settings such medicines may be unavailable, or unaffordably expensive for the majority of those in need of them. Furthermore, for many serious diseases (such as HIV/AIDS and tuberculosis) these essential medicines are protected by patents that permit the patent-holder to operate a monopoly on their manufacture and supply, and to price these medicines well above marginal cost. Recent international legal doctrine has placed great stress on the need to globalise intellectual property rights protections, and on the rights of intellectual property rights holders to have their property rights enforced. Although international intellectual property rights law does permit compulsory licensing of protected inventions in the interests of public health, the use of this right by sovereign states has proved highly controversial. In this paper I give an argument in support of states’ sovereign right to expropriate private intellectual property in conditions of public health emergency. This argument turns on a social contract argument for the legitimacy of states. The argument shows, further, that under some circumstances states are not merely permitted compulsory to license inventions, but are actually obliged to do so, on pain of failure of their legitimacy as sovereign states. The argument draws freely on a loose interpretation of Thomas Hobbes's arguments in his Leviathan, and on an analogy between his state of War and the situation of public health disasters.