The Altmark was a German fleet tanker and supply ship that had been used to support the Admiral Graf Spee on her commerce raiding sortie in the South Atlantic in 1939. Prior to the battle of the River Plate, the captain of the Graf Spee had transferred 299 British prisoners of war to the Altmark. After the battle the Graf Spee sailed to Montevideo, where she released her remaining prisoners. Through them the British government learnt of the remaining prisoners, and began to hunt for the Altmark. The initial search failed because the Altmark remained in the South Atlantic for two months after the destruction of the Graf Spee, only beginning her journey home on 22 January. She successfully cross the North Atlantic, and passed between the Faeroe Islands and Iceland without being discovered. However, on 15 February the British learnt that the Altmark had just passed Bergen, on the west coast of Norway. Under international law the British seamen onboard the Altmark should have been released into Norwegian custody when she stopped in Bergen. Instead, they were hidden below decks, in storage lockers and even an empty oil tank, while the Norwegians conducted two limited searches of the ship.
Fortunately the British had a small squadron patrolling off the Norwegian coast, under the command of Captain P. L. Vian of the destroyer Cossack. He was ordered to make the interception of the Altmark his first priority. At just after 1 pm on 16 February the Altmark was sighted by two separate Hudson aircraft from RAF Coastal Command. One hour later Captain Vian, at the head of a small flotilla of destroyers intercepted the German ship. The Altmark was in the company of two Norwegian destroyers, but despite this Vian ordered the destroyers Ivanhoe and Intrepid to board the Altmark. This first attempt failed, and the Altmark successfully made her way into Jössing Fjord, a mile and a half long steep sided fjord.
At 4.10pm Captain Vian followed the Altmark into the fjord. There he found the Norwegian torpedo boat Kjell, and demanded they order the Germans to hand over the prisoners. The Norwegian response was that the Altmark had been searched twice at Bergen, and had found to be an unarmed ship. She was therefore allowed to use Norwegian territorial waters. Vian withdrew outside territorial waters, and consulted with the Admiralty. At this point Winston Churchill, the First Lord of the Admiralty intervened. Churchill told Vian to tell the Norwegians that if they did not agree to escort the Altmark back to Bergen with a joint Anglo-Norwegian guard then he would seize the Altmark himself. Vian, on the Cossack re-entered Norwegian waters at 10pm. The Norwegians turned down the British request, and so Vian prepared to board.
The Altmark attempted to escape, and ran aground. Vian took the Cossack alongside, grappled with the Altmark, and boarded her in a way that would have been familiar to Nelson. After a brief fight the German crew either surrendered or escaped to land. The British then searched the Altmark, and much to their relief found the 299 British prisoners hidden below decks, as well two heavy and four light machine guns. Someone called out “The Navy’s Here!”, and the cry became famous in Britain. Inevitably the British action provoked a diplomatic protest from Norway, although both Britain and Germany had breached Norwegian neutrality. The general British attitude was that a failure to act would have set a dangerous precedent, suggesting that the Germans could use neutral waters for warlike purposes.