transferred to the training ship Niobe (1 July 1928 – 15 October 1928), attaining the rank of Seekadett (midshipman) on 11 October 1928. Following a 14-month stay on board the cruiser Emden (16 October 1928 – 3 January 1930) he advanced in rank to Fähnrich zur See (officer cadet) on 1 January 1930. Emden at the time was under the command of Lothar von Arnauld de la Perière, a U-boat commander during World War I. Hartenstein then underwent a number of officer training courses at the Naval Academy at Mürwik, including navigational training cruises on the survey vessel Meteor, before transferring to the light cruiser Köln (1 October 1931 – 23 September 1934). His stay on Köln was occasionally interrupted to attend further training courses at Wilhelmshaven and Kiel-Wik. During this assignment on 30 January 1933, the Nazi Party, under the leadership of Adolf Hitler, came to power in Germany, and began to rearm the navy. In 1935, the Reichsmarine was renamed the Kriegsmarine. Hartenstein served as first watch officer on the torpedo boat from 30 September 1936 to 13 November 1938. He was promoted to Kapitänleutnant (captain lieutenant) on 1 June 1937. Hartenstein participated in several patrols in 1937 and 1938 on board Greif in Spanish waters during the Spanish Civil War. For these services he was awarded the Spanish Cross in Bronze () on 6 June 1939. World War II At the outbreak of World War II, Hartenstein continued to serve on torpedo boats. In this position, he completed 65 patrols in the North Sea, Norwegian waters, the Bay of Biscay and the English Channel in the first one and a half years of the war. Hartenstein took command of torpedo boat on 20 November 1938. In October 1939 Hartenstein transferred and switched command of torpedo boat with Kapitänleutnant Franz Kohlauf who took over command of Seeadler. On 30 March 1941, command of Jaguar was given to Kapitänleutnant Friedrich-Karl Paul and Hartenstein transferred to the U-boat force, and on 4 September 1941 was given command of U-156, a Type IXC U-boat. For his service on torpedo boats, Hartenstein was awarded the German Cross in Gold () on 2 February 1942. Karl Dönitz personally pinned the award on Harteinstein's leather jacket on 17 March 1942. U-156 was first assigned to the 4th U-boat Flotilla at Stettin as a training boat, then was transferred to the 2nd U-boat Flotilla at