'DT' (i) was the British naval undertaking to find and board the 13,580-ton German oiler and supply vessel Altmark, and to rescue the British merchant marine prisoners the ship was carrying to Germany (16 February 1940).

Altmark had supported the 'pocket battleship' Admiral Graf Spee during the latter’s raid in the South Atlantic between September and December 1939, and seamen taken from the ships sunk by the pocket battleship had been transferred to Altmark. After Admiral Graf Spee had been scuttled by her crew in the Rio de la Plata estuary on 17 December 1939 in the aftermath of the 'Battle of the River Plate', Altmark returned toward Germany, steaming around the north of the UK and then to the south along the coast of neutral Norway.

The German ship was investigated three times on 15 February by officers from Norwegian vessels, who boarded the ship, carried out cursory searches and took the word of the Germans that the vessel was conducting purely commercial business. First the oiler was boarded by the torpedo boat Trygg off Linesøy, then by the torpedo boat Snøgg in the Sognefjord, and finally in person by Admiral Carsten Tank-Nielsen and the destroyer Garm in the Hjeltefjord. After the third boarding, Altmark was escorted to the south by the torpedo boats Skarv and Kjell and the guard boat Firern.

The British prisoners held in the ship’s hold reportedly made a major effort to make their presence known, but the German crew drowned the noise by running winches. However, the Norwegian search parties did not inspect the hold, and allowed the ship to continue on her way.

Altmark was spotted off Egersund later in the same day by a flight of three Lockheed Hudson twin-engined maritime reconnaissance aircraft from RAF Thornaby in the north-east of England. After being intercepted by Captain P. L. Vian’s destroyer Cossack, Altmark entered the Jøssingfjord, but the British destroyer followed her on the next day and forced the German ship aground. The British then boarded Altmark at 22.20 on 16 February, and after some hand-to-hand fighting, which cost the Germans four men killed and five wounded to one British sailor wounded, overwhelmed the ship’s crew and then went down to the hold, from which they freed 299 merchant seamen.

Cossack left the Jøssingfjord just after 24.00 on 16/17 February, and although the Norwegian escorts protested, they made no effort to intervene. The official explanation later given by the Norwegian government was that, according to international treaty, a neutral country was not obliged to resist a vastly superior force.

The British had planned to tow the German ship to a Scottish port, but the damage to the oiler’s stern frustrated this idea, and Altmark eventually reached Germany.

The Norwegians were angered that their neutrality had been infringed, but wished not to be dragged into the war. Nonetheless, the Altmark incident raised doubts in the minds of the Allies, and well as among the Germans, about the reality of Norway’s neutrality. Each side had contingency plans for military action against Norway, primarily to control the seaborne traffic in Swedish iron ore, on which the German armaments industry depended in the early stages of the war. The Altmark incident convinced Adolf Hitler that the Allies would not respect Norwegian neutrality, and on 19 February, he decided to intensify the planning for 'Weserübung', the seizure of Denmark and Norway starting on 9 April 1940.

The Altmark incident gave the British a short-term but much needed boost in morale during the period of the 'Phoney War' that lasted from the start of the war in September 1930 to the launch of 'Gelb' in May 1940. The incident also had a lasting propaganda effect in German-occupied Norway during the war, when the Norwegian collaborationist government tried to offset the nickname 'Quisling' for its adherents through the use of the location of the skirmish, Jøssingfjord, to coin the supposedly derogatory term 'Jøssing' for those who were pro-Allied. The attempt was wholly misconceived as 'Jøssing' was adopted as a positive term by the general public, and the word had been banned from official use by 1943.