On 1 January 1937 he was appointed "Oberfähnrich zur See". After returning to Germany in February 1937, Maus took several technical courses. On 1 April 1937 he was promoted to "Leutnant zur See" and returned to the cruiser "Nürnberg", where he served as adjutant from 3 March 1939 to 14 June 1939 and was promoted to "Oberleutnant zur See" on 1 April 1939. He was transferred to the battleship on 15 June 1939 and was present on the attacking battleship at the Battle of Westerplatte at the beginning of World War II. On 29 April 1940 he was transferred the "U-Boot-Waffe", and on 22 December 1940 he was assigned to , serving under the command of Karl-Friedrich Merten and participating in two missions of the U-boat. On 1 November 1941 he was promoted to "Kapitänleutnant" and assigned to the 24th U-boat Flotilla. After completing a training for U-boat commanders beginning on 20 April 1942 he was assigned to on 5 May 1942 and became her commander on 12 June 1942.