On the morning of February 16, 1940, the British destroyer HMS Cossack, under the command of Captain Philip Vian, conducted a daring operation against the German tanker Altmark, which was anchored in Norwegian waters. The Altmark had been involved in operations against Allied shipping, capturing hundreds of British sailors during the early months of World War II. By this time, the tanker had taken on a significant number of British prisoners, numbering 299, and was making its way back to Germany

When HMS Cossack confronted the Altmark, Norway was officially a neutral country, and the incident raised international tensions. Despite this, the Cossack's boarding party managed to surprise the German crew and successfully liberated the prisoners. During the boarding, a scuffle ensued, resulting in the death of a German crew member and several others being injured. The prisoners were found in dire conditions, and their release was met with jubilation in Britain, further fueling anti-German sentiment.

Following the operation, the British government justified the actions of HMS Cossack by stating that the circumstances warranted the response under international maritime law, as the Germans had violated neutrality by holding prisoners aboard their ship. The event was significant as it highlighted the escalating naval conflict between Britain and Germany early in World War II and served as a rallying point for British morale during a period of uncertainty in the conflict.