During the first two years of the war Prien was a U-Boat Commander who became a German national hero. As Commander of submarine U.47 he was ordered by Admiral Doenitz to plan and execute an attack on Scapa Flow, the British anchorage in Scotland. In the early morning of 14 October 1939 Prien’s U-Boat penetrated a narrow passage into the anchorage. Though most of the fleet had been removed to Loch Ewe, Prien managed to sink the HMS Royal Oak with 833 men on board. Occurring only six weeks after Britain had entered the war, it was a blow to morale but led to a tightening up of anchorage defense which was much needed.
Prien went on to fight in the Atlantic and Channel until on 7 March 1941 U.47 was sunk with all hands by the corvettes Arbutus and Camellia and destroyer Wolverine. The news of his death was kept secret for a long time because of his enormous popularity.