The use of disguised and armed merchant ships against the enemy's maritime communications had been a feature of German naval strategy in the First World War and was adopted again in the Second World War. The number of ships they sank and captured was insignificant when compared to the size of the Allied Merchant Marine and the losses inflicted by other forms of attack, especially the submarine. Nevertheless, for a relatively cheap investment of a moderately sized merchant ship and some semi-obsolete armaments drawn from reserve or from old warships, the Hilfskreuzer or auxiliary cruiser could force a quite disproportionate investment of resources by the Allies. Major warships whose presence was needed elsewhere had to be employed on long, tedious patrols, ships had to be re-routed and their sailings sometimes stopped, and choke-points continually swept for minefields laid by these auxiliary cruisers.

One such vessel was the Kormoran, originally launched in 1938 as the HAPAG freighter Steiermark. She was a new ship which had just run trials but had not yet made her maiden voyage. At 8,736 tons gross, with a length of 164 metres and a beam of 20.2 metres she was the largest and fastest of the vessels taken up for conversion to raiders. Her diesel electric propulsion gave her a maximum speed of 18 knots. At 10 knots she could cruise for nearly a year. She was armed with eight 15-centimetre guns, which had previously been mounted on old battleships, two 3.7-centimetre anti-tank guns and five 2-centimetre anti-aircraft guns. The guns were carefully hidden behind counterweighted screens which allowed them to be brought into action rapidly. On either side of the hull, just forward of the ship's superstructure, twin manually-trained torpedo tubes were concealed behind lifting flaps. These were supplemented by single fixed underwater tubes mounted port and starboard. The ship carried 320 mines, which were to be laid with the assistance of a launch, the LS 3. This launch which was also fitted to fire torpedoes. Two Arado 196 floatplanes completed the Kormoran's armament.

The new raider entered service on 9 October 1940 under the command of Captain Theodor Detmers. She carried out her working up trials in the Baltic and then departed Gotenhafen for the Norwegian port of Stavanger on 3 December 1940. From Stavanger she broke out into the North Atlantic through the Denmark Strait, disguised as a little known Russian merchantman. She was permitted to commence operations on 19 December. Her first victim was the Greek steamer Antonis, taken on 6 January 1941 in the central Atlantic. The Antonis surrendered immediately and sent no wireless message. Twelve days later the tanker British Union was attacked at night. The tanker transmitted a raider warning message and returned fire but was overwhelmed by the Kormoran's heavier armament. She caught fire and had to be sunk by torpedo. As the raider sped away from the scene, she narrowly missed interception by the armed merchant cruiser HMS Arawa, which had been fitted out in Australia and was crewed mostly by Australian reservists.

Captain Detmers now moved further south and operated in the narrowest part of the Atlantic between the bulge of Africa and Brazil. On 29 January the Kormoran fell in with the refrigerator ship Afric Star and sank her. Only two hours after the Afric Star had been despatched another vessel was sighted in the darkness and sunk. She was the freighter Eurylochus, carrying aircraft to Takoradi. Both ships were able to transmit warning signals.

On 7-9 February the Kormoran refuelled and re-provisioned from the supply ship Nordmark before moving to the South Atlantic, where she met the raider Pinguin on 25 February. The two captains exchanged notes, and the Pinguin passed 210 kilograms of white metal to be cast into bearings for the Kormoran's engines, which had been giving an inordinate amount of trouble. The ships parted the next day and the Kormoran returned to her area of operations.

As the raider sailed north the trouble with her engines persisted. She met the submarine U 124 on 15 March and and the armoured ship Admiral Scheer the next day. The submarine and the Kormoran took five days to transfer supplies due to adverse weather, but at least the raider was able to obtain 320 kilograms of white metal.

Continuing her patrol she sank the tanker Agnita on 22 March, her first victim for seven weeks. Three days later the large tanker Cawadolite was taken and sent to Bordeaux as a prize, the only ship attacked by the Kormoran which was not sunk. She also provided more white metal for the raider.

The Kormoran met the Nordmark and the submarines U 105 and U 106 on 27 March. Nordmark was unable to supply fuel so, after transferring stores to U 105, the raider parted company on 2 April to obtain fuel from another supply ship, the Rudolf Albrecht. After refuelling the Kormoran again met the Nordmark and transferred prisoners and submarine supplies to her.

Returning to operations, the Kormoran took the freighters Craftsman and Nicholas D.L. on 9 and 12 April respectively. On 19 April she met the raider Atlantis and the supply ships Alsterufer and Nordmark. After refuelling and re-ammunitioning she sailed southwards, round the Cape of Good Hope, entering the Indian Ocean on the night of 12 May. On 14 May she rendezvoused with the supply ship Alstertor and the tender Adjutant, refuelling both.

Operating south of Ceylon the Kormoran had no luck for a month so, on 21 June she entered the Bay of Bengal. A plan to mine the approaches to Madras came to nought when the raider was sighted by Allied shipping on 24 June but in the course of withdrawal she sank the Yugoslavian Velebit and the Australian Mareeba two days later.

The Kormoran moved to an isolated part of the Indian Ocean and between 2 and 17 July refitted herself. She then ranged the Indian Ocean without success until 26 September when she sank the Stamatios G. Embiricos. This was her last merchant victim. From 16 to 25 October she refuelled and re-provisioned from the supply ship Kulmerland in the south-eastern Indian Ocean and then proceeded to operate off Shark Bay.

On 19 November a cruiser was sighted . The Kormoran attempted to evade, but the cruiser, HMAS Sydney, had sighted her and closed, requesting her identity. Captain Detmers attempted to bluff his way clear, claiming that he was the Dutch ship Straat Malakka. When the Sydney demanded Straat Malakka's secret call sign, Detmers realised that the game was up and, at 5.30 pm, opened fire. The Sydney had closed to a range of about 1,200-1,500 metres and was quickly hit by the Kormoran's 15-centimetre guns and a torpedo. The closeness of the range allowed the rapid firing 3.7-centimetre and 2-centimetre guns to be used with considerable effect against the superstructure, torpedo tubes and secondary armament. The Australian cruiser was soon ablaze, with her forward turrets wrecked. Her after turrets engaged the Kormoran, hitting her in the engine room and causing a fire that was eventually to prove fatal. The Sydney steamed slowly away still under fire from the raider, which ceased fire at 6.25 pm.

The Kormoran was stopped and on fire. It became apparent that the ship could not be saved and Captain Detmers ordered her scuttled. She blew up and sank at 12.30 am on 20 November 1941. The number of survivors given in various sources varies from 315 to 317, plus three Chinese from the Eurylochus.