[3] Aged 20, Hartenstein re-applied for the cadetship, was accepted and began his naval career with the Reichsmarine on 1 April 1928 as a member of "Crew 28" (the incoming class of 1928).

He underwent basic military training in the 2nd department of the standing ship division of the Baltic Sea in Stralsund (1 April 1928 – 30 June 1928).

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).

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 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.

[Tr 4] The convoy received aerial protection from Messerschmitt Bf 109 fighter aircraft and split up once they reached Cuxhaven.

Hartenstein's orders were to conduct a simultaneous surprise attack of the Gruppe Neuland against the shipping traffic off Aruba and Curaçao.

At 01:31, U-156 surfaced in San Nicolaas Harbour some 1.5 km (0.81 nmi; 0.93 mi) offshore and attacked the two British tankers at anchor.

Loaded with crude oil, the steamer immediately burst into flames, killing eight of her 26 crewmen and wounding her captain Herbert McCall.

This achievement earned Hartenstein a reference on 6 June 1942 in the Wehrmachtbericht, a propaganda bulletin issued by the headquarters of the Wehrmacht, the OKW.

Prior to the departure Dietrich von dem Borne, who had been severely wounded on the second patrol, was replaced by Oberleutnant zur See of the Reserve Gert Mannesmann.

Clan Macwhirter was hit by two torpedoes and sank, killing nine members of the crew and two gunners; 79 sailors survived the sinking.

[22] On 12 September 1942 U-156 was patrolling off the coast of West Africa midway between Liberia and Ascension Island roughly 600 nautical miles (1,100 km; 690 mi) south of Cape Palmas.

[21] He closed, hoping to capture the ship's senior officers, but then saw over two thousand people, many of them Italian prisoners of war, struggling in the water.

[24] At 01:25 on 13 September 1942 Hartenstein radioed the Befehlshaber der U-Boote (BdU—commander of U-boats) requesting guidance and confirmation on how to proceed.

[27] Heading to a rendezvous with Vichy French ships under Red Cross banners, the U-boats were attacked by a U.S. Army B-24 Liberator bomber (343d Bomb Squadron; Lieutenant James D. Harden) at 12:32 on 16 September 1942.

[28] The attack ordered by Captain Robert C. Richardson III, which killed a number of people in the lifeboats and damaged U-156, forced Hartenstein to abandon the rescue operations.

[29] A majority of survivors were later rescued by British merchant ships and two Vichy French warships, the cruiser Gloire and the sloop Annamite, out of Dakar, Africa.

[4] Hartenstein issued a bottle of beer to each member of the crew and held a speech honouring the achievements of everyone on board, and telling them that he would wear the decoration in their name.

[32] On 19 September 1942, U-156 was roughly 800 nautical miles (1,500 km; 920 mi) south of Freetown and the crew was still repairing minor damage, when the lookout spotted a ship at 04:30.

This account of that attack and the impression that the humanitarian actions of Hartenstein made is documented in the book by David Cledlyn Jones, The Enemy We Killed, My Friend.

[34] Jones himself disagrees, stating that Hartenstein was concerned about the survivors' well-being but did not inquire about provisions, nor did he offer additional food or water.

Hartenstein did express that it would have been his wish to tow them at least some distance to the African coast, but explained that he was not able to do so as he recently had been attacked while attempting to aid survivors.

Korvettenkapitän Ernst Kals, chief of the 2nd U-boat Flotilla at Lorient, sent a letter to Hartenstein's parents on 23 April 1943 indicating that their son had been posted as missing in action as of 12 March 1943.

The service was attended by his parents, his sisters and other members of the family, the mayor of Plauen, Eugen Wörner, senior officials and councillors.

The local press reported that "His parents have accepted that their loving son will not return home but is resting in peace with his Lord.

[39] As commander of U-156 Werner Hartenstein is credited with the sinking of 20 ships (including the motor boat Letitia Porter on board Koenjit) for a total of 97,504 gross register tons (GRT), further damaging three ships of 18,811 GRT and damaging one warship, USS Blakeley, of 1,190 long tons (1,210 tonnes).