No two nations have exchanged natural resources, produced transborder environmental agreements, or cooperatively altered ecosystems on the same scale as Canada and the United States. Environmental and energy diplomacy have profoundly shaped both countries’ economies, politics, and landscapes for over 150 years. Natural Allies looks at the history of US-Canada relations through an environmental lens. From fisheries in the late nineteenth century to oil pipelines in the twenty-first century, Daniel Macfarlane recounts the scores of transborder environmental and energy arrangements made between the two nations. Many became global precedents that influenced international environmental law, governance, and politics, including the Boundary Waters Treaty, the Trail Smelter case, hydroelectric megaprojects, and the Great Lakes Water Quality Agreements. In addition to water, fish, wood, minerals, and myriad other resources, Natural Allies details the history of the continental energy relationship – from electricity to uranium to fossil fuels –showing how Canada became vital to American strategic interests and, along with the United States, a major international energy power and petro-state. Environmental and energy relations facilitated the integration and prosperity of Canada and the United States but also made these countries responsible for the current climate crisis and other unsustainable forms of ecological degradation. Looking to the future, Natural Allies argues that the concept of national security must be widened to include natural security – a commitment to public, national, and international safety from environmental harms, especially those caused by human actions.