Without this capture, the war could have been prolonged for months, maybe even years:
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.
But breaking the German naval code, made possible in large part by the recovery of U-110's machine, provided the British with a leg up at a time when the war at sea was very much in doubt.
The capture of a U-boat on the high seas was a rare and considerable achievement, since submarine crews scuttled their boats rather than let them fall into enemy hands. In this case, the U-boat’s commander, Kapitänleutnant Fritz Julius Lemp, thinking he was going to be rammed by an oncoming destroyer, ordered his crew to abandon ship. (His precise order, according to one survivor, was 'Last stop. Everybody off.') Seeing the Germans leaving the boat, the British commander managed to veer away and avoid a collision.
Lemp, already in the water when he realized his boat wasn't going to be rammed, was swimming back to U-110 to scuttle her when he was either shot by the British (according to the Germans) or simply disappeared (according to the British).
Three other U-boats were captured at sea during the war, most notably the U-505, surprised by an American task force off the African coast in June 1944. That boat is on permanent display at the Museum of Science and Industry in Chicago.
Pop culture footnote: The thoroughly mediocre movie, U-571, was loosely based -- very loosely based -- on the capture of U-110. It was also shot through with historical inaccuracies, but that's a subject for another time and place.
(Source: Uboat.net, Wikipedia)
(Via Wired News.)