On board the Altmark were roughly 300[a] Allied prisoners (officially internees), whose ships had been sunk by the pocket battleship Graf Spee in the Southern Atlantic Ocean.

British naval forces cornered the tanker, and later the destroyer HMS Cossack boarded the Altmark near the Jøssingfjord and liberated all the prisoners.

The German government claimed that the boarding had been a violation of international law and of Norwegian neutrality and later used the incident in the propaganda broadcasts of "Lord Haw-Haw".

In each instance, the men who boarded the ship carried out cursory searches and took the Germans' word that the vessel was conducting purely commercial business.

The Altmark's Norwegian naval escorts blocked initial attempts to board the ship, and aimed their torpedo tubes at the Cossack.

Captain Vian then asked the British Admiralty for instructions, and received the following orders directly from the First Lord of the Admiralty, Winston Churchill: Unless Norwegian torpedo-boat undertakes to convoy Altmark to Bergen with a joint Anglo-Norwegian guard on board, and a joint escort, you should board Altmark, liberate the prisoners, and take possession of the ship pending further instructions.

[6] The Norwegian naval forces refused to take part in a joint escort and reiterated that their earlier searches of Altmark had found nothing.

[9] However, Jim Rhodes, a former crew member of Cossack, wrote in the April 2002 newsletter of the Association that he had witnessed a cutlass being carried by one of the boarding party.

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 incident also had a lasting propaganda effect in German-occupied Norway during the war, when the Norwegian collaborationist government tried to neutralise its nickname "quislings" by using the location of the skirmish, Jøssingfjord, to coin the derogatory term "jøssing", referring to pro-Allies and anti-Nazis.

[11] The phrase "the Navy's here" became well known in Britain and was used as the title of a book about the incident;[7] the publisher referred to "the simple statement which stirred the imagination of the world".

[12] A popular song was written by Ross Parker and Hughie Charles and saluted the incident by comparing it with those of Drake, Nelson, Beatty and Fisher.