'Orion' was a German naval and merchant navy operation in the Baltic Sea to transport refugees from Gotenhafen (Gdynia) in Poland to Copenhagen in German-occupied Denmark (30/31 January 1945).

The primary casualty of this undertaking was the 25,484-ton liner Wilhelm Gustloff. Launched in May 1937, the ship had been requisitioned into naval service on 1 September 1939 and served as a hospital ship in 1939/40. Beginning on 20 November 1940, she was stripped of medical equipment, repainted in standard naval grey and assigned as a floating barracks for about 1,000 personnel of the 2^nd Unterseeboot-Lehrdivision at Gotenhafen near Danzig. The ship remained here for the next four years, but late in 1944 she was put back into service to transport civilians and military personnel as part of 'Hannibal'.

The ship’s final voyage was in the 'Orion' undertaking and designed to evacuate German civilians, sailors and soldiers from Gotenhafen as the Soviet forces advanced on the port. The ship’s complement and passenger lists included 6,050 named persons, but not the many civilians who boarded the ship without being recorded in the ship’s official embarkation records. A German historian later assessed the total as 10,582 persons in the form of 173 crew, 918 officers, non-commissioned officers and men of Kapitän Karl Neitzel’s 2^nd Unterseeboot-Lehrdivision, 373 female naval auxiliary helpers, 162 wounded soldiers and 8,956 civilians. Although the ship was built for 1,465 passengers, she had the capacity to board many more for a short trip by making use of her public recreation spaces for temporary accommodation, but she was carrying less than 50% of the survival equipment necessary for the extra passengers.

The ship departed Gotenhafen early on 30 January, accompanied by the 20,815-ton passenger liner Hansa, also filled with civilians and military personnel, and two torpedo boats. Hansa and one of the torpedo boats then developed mechanical problems and could not continue, leaving Wilhelm Gustloff to proceed under escort of just one torpedo boat, Löwe. The ship had four captains (three civilian and one naval) on board, and these could not agree on the best course of action to guard against submarine attacks. Against the advice of the naval commander, Korvettenkapitän Wilhelm Zahn, a submariner who argued for a course in shallow waters close to shore and without lights, the senior civilian captain, Friedrich Petersen, decided to head for deep water. When he was informed by radio of an oncoming German minesweeper convoy, he decided to activate his ship’s navigation lights so as to avoid a collision in the dark despite the fact that this made his ship more readily visible to foes as well as friends. As her equipment included anti-aircraft weapons, the ship had been steaming without lights.

The ship was soon sighted by the Soviet submarine S-13, under the command of Kapitan 3 ranga Aleksandr Marinesko, who ordered three torpedoes to be fired at Wilhelm Gustloff's port side at a point some 18.5 miles (30 km) offshore between Grossendorf and Leba. All three weapons struck the ship: the first impacted near the bow, the second just forward of midships, and the third the engine room in the area below the ship’s funnel, immediately cutting all electrical power.

The ship soon took a list to starboard and started to settle by the head. In the panic which followed, many of the passengers were trampled in the rush to the lifeboats and life jackets. The water in this part of the Baltic Sea temperature is normally about 4° C (39° F), but this was a particularly cold night, with an air temperature as low as -18° C (-0.4° F) and ice floes covering the surface. Many deaths were caused either directly by the torpedoes or by drowning in the onrushing water. Others were crushed in the initial panic on the stairs and decks, and many jumped into the icy Baltic.

Fewer than 45 minutes after being struck, Wilhelm Gustloff went down in 140 ft (45 m) of water.

German vessels were able to rescue some of the survivors from the attack: the torpedo boat T 36 rescued 564 people, the torpedo boat Löwe 472, the minesweeper M 387 98, the minesweeper M 375 43, the minesweeper M 341 37, the steamer Göttingen 28, the torpedo-recovery boat TF 19 seven, the freighter Gotland two, and the patrol boat V 1703 one baby. Thus the overall losses may have been as great as 9,343 men, women, and children, which would make this the largest loss of life in a single sinking in maritime history.