After World War I, the German admirals were shocked to discover that the Royal Navy had had an immense advantage—they had penetrated the secret German radio codes. Confronted with this fact, the Germans determined that any future codes would need to be so complex that breaking them would, theoretically, be impossible. The result was the famous Enigma machine, whose settings were altered so frequently and variables so great that the Germans believed capture of a machine by the enemy would not compromise Enigma for long. But the British proved them wrong. They managed to obtain the machine and determine the method of setting the code, giving the Allies a critical tactical advantage. As the author points out, this intelligence triumph was particularly important during the Battle of the Atlantic, when Hitler's U-boat wolf-packs were wreaking havoc on the convoy lifelines to the British Isles. Without the decoded Enigma messages, the Allies' narrow victory would likely have turned to defeat. This skillful analysis of Enigma's development and its role during U-boat operations includes details of Allied boardings of U-boats from which Enigma machines could have been captured. It is one of the greatest espionage stories in the history of naval warfare.