Chapter 5 explores the threat that the Cold War posed to postwar internationalism by narrating how the United States, Canada, and Great Britain sent scientists back to Grosse Île in 1949 to undertake new explorations of turning rinderpest into a biological weapon. This was the darker side of the growing sense of environmental interconnectedness. Efforts to research rinderpest as a biological weapon continued on Grosse Île until 1957, after which they were moved to Plum Island, but they remained limited. The Cold War threatened the international eradication effort, but it did not destroy it. Work continued despite and outside of that global conflict. Following that story reminds readers of the limitations of the Cold War framework as a way of making sense of international relations in the second half of the twentieth century.