In this paper, I argue that a new principle of background justice should be added to Rawls's Law of Peoples because climate change is an international and intergenerational problem that can destabilize the Society of Peoples and the well-ordered peoples therein. I start with explaining the nature of my project and Rawls's conception of stability. I argue that climate change poses a realistic threat to the stability of climate-vulnerable liberal peoples and as a result undermines international peace and security. Despite the uncertainties due to the complexity of the climate system and about the resilience of liberal societies, liberal peoples' fundamental interests in just basic institutions and national security call for the adoption of a precautionary principle. Rawls's own principles are, I argue, inadequate to solve the stability problem from climate change. Still, his framework provides the theoretical resources to develop a new extension. I propose a new Rawlsian principle of international, intergenerational justice that guarantees the environmental background conditions under which well-ordered peoples can sustain their basic structure over generations and sketch the principle's institutional implementation. I conclude with the theoretical and practical significance of this extension of Rawls's theory.