On 4 July 1916, during World War I, Stummel volunteered for the Imperial German Navy, with the prospect of a naval officer career.

After attending a briefing for five months, he was posted to the Deutschland-class pre-dreadnought SMS Hannover and promoted to Bootsmannsmaat (boatswain's mate) on 27 January 1917 and Fähnrich zur See on 26 April 1917.

Naval Brigade, before being transferred on 30 June 1920, for a few months to the Ships Cadre Detachment of the Baltic Sea as a platoon leader.

He was then posted for eight months as radio and watch officer on the cruiser and coastal defense ship SMS Thetis, remaining in the position until February 1924.

In April 1929 Stummel underwent further training at the Königlich Technische Hochschule Charlottenburg, and promoted in August 1929 to Kapitänleutnant, until March 1931 before being assigned as a radio officer to the battleship SMS Schleswig-Holstein.

The second was German submarine U-33 then captained by Hans-Wilhelm von Dresky, which was sunk in February 1940 in shallow water in the Firth of Clyde.

The third event occurred four days later, when the German tanker, the Altmark, a supply ship for the German cruiser Admiral Graf Spee, that was carrying British prisoners of war, was attacked in the neutral waters of the Norwegian fjord, and boarded by British sailors, who shouted, The navy's here, and subsequently freed the prisoners.

Stummel spent several weeks on the investigations, speaking principally to the cryptanalysts and cipher specialists, but did not conclude that a leak had occurred in any of those cases.

In his report, Stummel stated in the conclusions, that: In early May 1943, Dönitz fired Erhard Maertens, for reasons that went beyond his fears about crypto-security and sent him to run the Kriegsmarine shipyard in Kiel.

[4] Despite the claims, and perhaps nagging doubt that the Enigma machine cipher was insecure, in 1944, Stummel convinced Dönitz to carry the Kriegsmarine basic cryptography principles to its logical conclusion.