Doenitz specialised in submarine warfare from 1916 and in the 1936-39 period built up the German U-Boat fleet. He played a key role in the Battle of the Atlantic during the Second World War, devising the tactic of hunting allied vessels in packs. He replaced Admiral Raeder as Commander in Chief of the German Navy in 1943 and retained the position until 1945. Hitler’s final testament appointed Doenitz as President of the German Reich. He was sentenced to ten years imprisonment at the Nuremberg Trials and released from Spandau in 1956.