He was Admiral Hipper’s chief of staff in World War I, and from 1928 was commander-in-chief of the German navy, secretly rebuilding it in violation of the Versailles Peace Settlement. In the 1930s he elaborated his ‘Z-Plan’ for building a fleet capable of challenging Britain, but World War II began before this was achieved. He resigned and was replaced by Doenitz in January 1943, after Hitler became outraged by the poor performance of the surface fleet against Allied convoys. His part in unrestricted U-boat warfare led to his post-war imprisonment after trial at Nuremberg.