The fall of 1941 was the darkest time for Britain. The island nation stood alone. Hitler had conquered Europe and the United States had yet to join the war. The Nazi strategy to bring the Brits to their knees was simple: Starve them.

Winston Churchill, the prime minister, faced a brutal reality: The battlefield that would determine whether or not Britain could survive was not on land nor, any longer, in the air, but on the Atlantic Ocean.

The nation’s food supplies had dropped by half. Each month half a million tons of shipping went to the bottom of the Atlantic, blasted there by torpedoes from the packs of German U-Boats hunting convoys from North America, much of the cargo essential supplies like wheat and meat.