From 1942 to 1944 fifteen German submarines carried out extended combat missions in the St. Lawrence. They destroyed or severely damaged 27 ships, including three Canadian warships, a U.S. Army troop transport, and the Newfoundland ferry Caribou. One ship went down near Rimouski, Quebec, less than 300 kilometers from Quebec City. More than 250 lives were lost, many of them women and children. It was the only battle of the twentieth century to take place within Canada’s boundaries, and the only battle to be fought almost exclusively by Canadian forces under Canadian, rather than Alliance, high command. The battle took place during moments of grave political crisis for prime minister William Lyon Mackenzie King, and directly influenced his government’s leadership of the war effort. And for over 40 years the battle was characterized as a Canadian defeat. Yet was it a defeat? No one looked at the classified wartime records until the 1980s, and the first full account to draw on them appeared only in 2003. Canada, those records suggested, mounted a successful defense with far fewer resources and in the face of much greater challenges than previously known. The book presents new material from wartime records—including “ultra” top secret Allied decryptions of German naval radio communications. It draws vivid pictures of the intense combat on Canada’s shores, the sailors and airmen who stretched shoe-string resources to the limits and beyond, and the interplay of the St. Lawrence battle with war politics in Ottawa, Washington, and London.