It was probably the most important battle of the Second World War. The German attempt to stem the vital trans-Atlantic flow of men and supplies to Britain would cost the Allies more than 2,600 merchant ships, 30,000 merchant seamen's lives, 5,000 aircrew, and about 2,500 aircraft. The battle ranged across the ocean: from the waters of the St. Lawrence, through the treacherous open sea of the North Atlantic, to the fjords of Norway and the Bat of Biscay...