After making two voyages as an unarmed merchantman, she was taken over by the German Imperial Navy on 19 February 1917 and converted into U-155, armed with six torpedo tubes and two deck guns.

Deutschland was one of seven submarines designed to carry cargo between the United States and Germany, through the naval blockade of the Entente Powers.

Britain and France soon protested against the use of submarines as merchant ships, arguing that they could not be stopped and inspected for munitions in the same manner as other cargo vessels.

At 1:20 a.m. on 9 July Cape Henry was sighted and contact made with the Eastern Forwarding Company tug Thomas Timmins which had been specially altered to tow Deutschland alongside and been waiting some days.

[9] During their stay in Baltimore, the German crewmen were welcomed as celebrities for their astonishing journey and even taken to fancy dinners and an impromptu volksfest in the southwest part of the city.

The successful completion of this first voyage was commemorated with a tongue-in-cheek medal created by German artist Ernst Zehle, with a dedication to the British Lord Robert Cecil, responsible for the blockade, on the front.

At the same time the submarine U-53 also crossed the Atlantic to visit Newport, Rhode Island, and sank five Allied cargo ships just outside US territorial limits before returning home.

On 17 November as she was putting to sea, Deutschland accidentally rammed the tugboat T. A. Scott, Jr., which turned across her path suddenly while escorting her from New London to the open ocean.

[14] A third voyage, planned for January 1917, was aborted as German-US relations had worsened following the sinking of shipping bound for the United Kingdom, often just outside US territorial waters.

While the raid was light in damage (it killed four people), it alarmed Allied naval authorities about the defenseless nature of the Azores and their possible use as a base by boats like U-155 in the future.

Studt's orders directed him to cruise off the US coast in the region of the Nantucket lightship and lay mines off St. John's, Newfoundland and Halifax, Nova Scotia.

On U-155's outbound voyage she had captured and scuttled the Portuguese sailing ship Gamo, had attempted an attack on SS France, and destroyed by gunfire the Norwegian Stortind.

On 13 September U-155 engaged in another gun fight with the British merchantman Newby Hall, which managed to damage the submarine, denting her armour and causing serious leaks in her pressure hull which made diving temporarily impossible.

There, she was sold on 3 March 1919 to James Dredging Co. for £3,500, and then rapidly sold-on to Noel Pemberton Billing for £17,000, and then to John Bull Ltd (£15,000), a vehicle for Horatio Bottomley, who demilitarised the boat and embarked on a commercial tour that began at Great Yarmouth in September 1919, with the vessel re-christened Deutschland.

At the end of the tour, in June 1921 she was taken into dock for stripping at Clover, Clayton Birkenhead, where, on 17 September 1921, an explosion in the engine room killed five apprentices.