Prior to 1945, General Andrew George Latta McNaughton had already made a name for himself as an army general, engineer, inventor, and cabinet minister. After 1945, McNaughton occupied a number of key international roles for Canada: at the United Nations, on the Permanent Joint Board on Defence, and on the International Joint Commission. Even though he became one of Canada’s most important diplomatic actors during the early Cold War period, this aspect of his career has been mostly ignored by international historians. This article examines McNaughton’s key involvement in the evolution of a number of Canada–US water megaprojects, arguing that his nationalism underpinned his approach to bilateral relations, which combined deep technical expertise with a willingness to publicly assert the Canadian national interest. McNaughton’s approach should be studied not only to better understand North American environmental diplomacy in the Cold War but also to draw from it several lessons for contemporary times.