Canada emerged from the Second World War as a hydro-electric superpower. Only the United States generated more hydro power than Canada and only Norway generated more per capita. Allied Power is about how this came to be: the mobilization of Canadian hydro-electricity during the war and the impact of that wartime expansion on Canada’s power systems, rivers, and politics. Matthew Evenden argues that the wartime power crisis facilitated an unprecedented expansion of state control over hydro-electric development, boosting the country’s generating capacity and making an important material contribution to the Allied war effort at the same time as it exacerbated regional disparities, transformed rivers through dam construction, and changed public attitudes to electricity though power conservation programs. An important contribution to the political, environmental, and economic history of wartime Canada, is an innovative examination of a little-known aspect of Canada’s Second World War experience.