U-boat

Submarines had come close to starving Britain in the First World War, and the Treaty of Versailles banned Germany from having them. Hitler resumed building them in 1935, and Karl Dönitz, commander of the U-boat arm, developed ‘wolf pack’ tactics: groups of boats attacked convoys together, at night and on the surface. In 1940–1942 these inflicted huge losses on the Allies.

Radar, long-range aircraft, escort carriers, radio direction-finding and the reading of Enigma-encrypted signals turned the battle: in ‘Black May’ 1943 Dönitz withdrew his U-boats from the North Atlantic. By the end of the war Germany had commissioned more than a thousand submarines, about two-thirds of which were lost; most of their crews died.