Amid a growing emphasis on the peace–sustainability nexus, environmental cooperation remains underexplored as a diplomatic tool, especially in settings of deep-rooted conflict. This chapter combines environmental peacebuilding (EP) with the multiple streams framework (MSF) to explain how the USSR and Japan signed an environmental accord in 1991, despite the absence of a peace treaty and an unresolved territorial dispute. Drawing on archival sources, it reconstructs the changes that took place in parallel in the domestic environmental policy of the USSR and Soviet–Japanese dialogue. MSF’s convergence lens identifies political receptivity as the critical barrier; the case illustrates how environmental cooperation can emerge even in conditions of virtually no trust. This analysis further shows that, though environmental issues can be politicized, they may be reframed as shared challenges fostering gradual trust-building. Findings underscore that environmental initiatives can offer a constructive platform for engagement, even amid diplomatic stalemates and unresolved legacies. By integrating EP concepts with MSF, this study illuminates how environmental agendas can create policy windows for collaboration, providing new insights into environmental dialogue’s potential for advancing peace and sustainability.