The worst month was November 1942, when 721,700 tons were sunk by U-boats. At that point, the Kriegsmarine had more than 100 submarines operating in the Atlantic and, for most of the year, British code-breakers had been unable to decrypt key enemy signals because the Germans had added a fourth wheel to their Enigma machines. No wonder that the Allied Combined Chiefs of Staff stated after their Casablanca conference in January 1943, “Defeat of the U-boat must remain a first charge on the resources of [the] United Nations.”

During the course of that year, however, the situation gradually improved. The fourth wheel was cracked.