By May 1941, Great Britain was slowly being strangled and was down to less than six months of the food and essential supplies required to continue the war. In his memoirs, Winston Churchill would say that the only thing that really scared him during the Second World War was the U-boat threat.

On the morning of 9 May 1941, the German submarine U-110 was part of a Wolfpack that attacked a convoy south of Iceland. One of the escorts, HMS Bulldog, depth charged U-110 and severely damaged it. On the second pass, the Bulldog dropped more depth charges below the U-boat and forced it to the surface.

The German crew abandoned the sub, thinking it was going to sink. But it didn’t. The crew tried to re-board but some machine gun fire from a quick thinking sailor on the Bulldog convinced most of them of the folly of that action. None the less, the U-boat captain died trying: he assumed there was no need so he didn’t destroy the cipher books, message logbooks, or the Enigma machine, used to decode messages from fleet headquarters, before abandoning ship. A boarding party from the Bulldog recovered it all.

The capture of U-110 was an intelligence bonanza for Dr Alan Turing and the British code breakers at Bletchley Park. The Allies were already reading the Luftwaffe’s mail, within the month they were also reading the Kriegsmarine’s. The combination of the two allowed the British to reroute convoys away from German reconnaissance planes, surface raiders, and U-boats, not to mention target Rommel’s supply convoys between Italy and Libya. Merchant marine losses dropped significantly. It was the turning point in the Battle of the Atlantic that year.