May 9, 1941, British Capture Enigma Machine

Brad DeLong, quoting Wired:

May 9, 1941: German Sub Caught With the Goods: 1941: British destroyers capture a German submarine, U-110, south of Iceland. The British remove a naval version of the highly secret cipher machine known to the Allies as Enigma, and then they let the boat sink -- to keep the fact of their boarding secret. The Enigma machine, used by the Kriegsmarine to encode and decode messages passing between shore command and ships at sea, was taken to Bletchley Park in England, where cryptographers including computer pioneer Alan Turing succeeded in breaking the naval code. The Germans, assuming U-110 had foundered with her secrets intact, failed to realize that their code was broken. The subsequent information passing before British eyes helped the Allies enormously in the Battle of the Atlantic.

Several versions of the Enigma machine existed, but the working principle -- a rotor system activated using a keyboard -- was the same. The machine itself had been around since the early 1920s and was used by other nations, too, although it is most closely associated with Nazi Germany. The Enigma used by the German army was decrypted as early as 1932 by Polish cryptographers, who later passed their methodology along to the British and French. In light of subsequent events (the Germans drove a Franco-British expeditionary force out of Norway and then crushed the French in a six-week campaign in 1940), the military value of this early intelligence is debatable.