[2] Under the command of Kapitän zur See Günther Lütjens, Topp sailed on Karlsruhe's fourth training cruise.

During this time at the naval academy, he advanced in rank to Fähnrich zur See (officer cadet) on 1 July 1935 and underwent further training.

[7] Topp made the acquaintance of Martin Bormann, Hitler's personal secretary and head of the Nazi Party Chancellery.

[10] One analyst wrote, "the contrasts and Topp's individual course make obvious the limitations of any quantitative study" [of his political convictions].

[5] After the war, Topp expressed his view that the German naval command, and his superior Karl Dönitz, knew of the Holocaust.

"[14] He presented Topp with his accusation, underlined, and a letter from Samuel Eliot Morison, the official historian of the United States Navy, which accepted Dönitz's defence that he knew nothing of the Nazi crimes.

[15] For his service on U-46, Topp was awarded the U-boat War Badge (U-Boot-Kriegsabzeichen) on 7 November 1939 and the Iron Cross 2nd Class (Eisernes Kreuz zweiter Klasse) in January 1940.

[18] Following the loss of U-57 — the boat was later raised and used for training purposes — Topp was awarded Iron Cross 1st Class (Eisernes Kreuz erster Klasse) and initially remained with 1st U-boat Flotilla.

On 4 November, he was sent to Blohm & Voss, the shipbuilding works in Hamburg, for construction training of U-552, a Type VIIC U-boat.

Following sea trials and training, Topp, with Leutnant zur See Siegfried Koitschka as his second watch officer, took U-552 on its first war patrol on 13 February 1941.

[21] The third ship sunk during Topp's second patrol in command of U-552, was the troopship S.S. Nerissa (5,583 GRT, 207 casualties and 84 survivors) on 30 April 1941 about 140 nautical miles west of the North Channel.

This sinking resulted in the third-largest loss of life for a ship sunk by U-boats in the approaches to the British Isles during the Second World War.

[30] The destruction of the Reuben James facilitated a worsening of already rapidly deteriorating diplomatic relations between Nazi Germany and the still nominally neutral United States of America.

President Franklin D. Roosevelt used the incident to shift public opinion toward a confrontational stance with the European Axis powers.

On 13 November 1941, amendments to the act allowed for the arming of US merchant ships, their operation in the war zone, and active assistance to the British Empire to increase the tonnage available to it.

[31] Hitler and the Nazi leadership wished to keep the US neutral and the order to minimise contact at sea remained in force, at least until Operation Barbarossa had destroyed the Soviet Union.

[31] Roosevelt did not publicly mention the destroyer was escorting a British convoy, was not flying the Ensign of the United States, and was in the process of dropping depth charges on another U-boat when it was engaged and sunk.

Codenamed Operation Drumbeat (Unternehmen Paukenschlag), the U-boat fleet inflicted the largest naval defeat on the US Navy in history.

[33] The American failure to initialise a blackout, ship captains' insistence on following peace-time procedures, and lack of effective naval defences contributed to high losses.

[38] Oberleutnant zur See Albrecht Brandi joined Topp's crew as a commander in training on U-552's seventh war patrol.

The patrol to the West Atlantic, Grand Banks of Newfoundland, and Nova Scotia started on 25 December in Saint-Nazaire.

Unable to load the spare torpedoes from deck stowage due to icing and inclement weather, BdU called him home.

On 11 April, Karl Dönitz also awarded him the U-boat War Badge with Diamonds (U-Boot-Kriegsabzeichen mit Brillanten).

When he spotted the coastal steamer off Chincoteague, Virginia, on 3 April 1942, he surfaced U-552, overtook it from astern, and, without offering the captain the chance to surrender, attacked it with his deck gun from 600 yards, firing a total of 93 rounds.

That day, he also received a preferential promotion to Korvettenkapitän (corvette captain) and Dönitz presented him an honorary dagger of the Kriegsmarine with diamonds.

On 1 October 1966, Topp was named deputy Inspector of the Navy and at the same time became the chief of the Führungsstab der Marine in the Federal Ministry of Defence.

For his service with the Bundesmarine, Topp was awarded the Commander's Cross of the Order of Merit of the Federal Republic of Germany (Großes Verdienstkreuz des Verdienstordens der Bundesrepublik Deutschland) on 19 September 1969.

[51] He was interviewed on World War II submarine operations for the Nova (TV series) special Hitler's Lost Sub, which detailed the efforts of a team of divers, led by John Chatterton and Richie Kohler, to identify an unknown German U-boat wreck 65 miles off the coast of New Jersey; the wreck was identified as U-869.

[52] A large oil painting of his deceased close comrade Endrass hung in his home after the war until the time of his death.

In his front room overlooking the Rhine River was the top of the periscope from U-552, which immediately after the war, some of his crewman had managed to remove from the captured boat and pass to him as a keepsake.