T he May 24, 1943, wireless message, encrypted and transmitted from U-boat headquarters outside Berlin, was intercepted at a time when the Allies at best held a tenuous grasp on Germany’s naval Enigma cipher system. Yet almost miraculously, experts at Bletchley Park, Britain’s code and cipher facility northwest of London, promptly cracked it.
Grand Admiral Karl Dönitz, former U-boat fleet commander and, since January, Germany’s overall navy chief, had ordered 16 submarines, then prowling North Atlantic convoy routes, to proceed to “naval grid square 87 of the large square West of TT.” As customary, the decrypted message was teleprinted to the Royal Navy’s Submarine Tracking Room and to F-21, the U.S. Navy equivalent.
The message’s top-secret content, what the Allies called “special intelligence” or “Ultra”— both meaning intelligence derived from solving high-grade codes and ciphers—reached Commander Rodger Winn and Lieutenant Kenneth Knowles. Winn of the Royal Navy and Knowles of the U.S. Navy were both middle-aged reservists— so-called “civilians in uniform.” Save for wartime manpower exigencies, neither would be serving, even in shore billets. But now Winn, stationed in London, and Knowles in Washington, D.C., had to unravel what amounted to a puzzle-within-a-puzzle—one with life-and-death consequences.
German naval charts were overlaid with a system of lettered and numbered grids. U-boats maneuvered according to grid positions, not latitude and longitude. Culling and cross-checking various sources had enabled Allied submarine trackers to piece together much of the Atlantic Ocean grid system. But the Germans greatly complicated matters, using two letter “substitution codes” that changed frequently. In this case what did the grid reference “TT” stand for? Failure, or even delay, in unlocking the U-boats’ precise destination—and intentions—could spell disaster in the Atlantic.
WITH ALLIED NAVY REGULARS needed at sea, filling backroom intelligence slots with reservists was the only option. But harnessing the disparate skills of attorneys, bankers, journalists, linguists, and mathematicians produced unanticipated benefits. Unburdened by chain-of-command rigidity, outsiders could, as one British observer quipped, “resist...cooking raw intelligence to make
