On July 25, 1939, six weeks before the German invasion of Poland and the outbreak of World War II, three British Secret Service men met with their Polish counterparts in an underground room deep in the Molokov-Pyry forest outside Warsaw. There the Poles offered the British the Third Reich's most precious secret: the phenomenally complicated Enigma machine that encoded and decoded all German military messages. It was a gift that would change the course of the impending war. By the end of the following day the machine was in England, together with everything the Poles had discovered about its operation. The British immediately established a special center at Bletchley Park, a rambling country mansion, and gave the operation the code name Ultra. Soon it became apparent just how complex the Enigma machine really was. Battery-powered to facilitate complete mobility on the battlefield, the Enigma machine resembled an electric typewriter. But when one key was pressed on the keyboard, a complex system of three parallel rotating wheels went into action. Each wheel displayed a full alphabet. The accompanying plugboard, similar to a telephone switchboard, ensured that a different letter would light up on a panel containing 26 bulbs - one for each letter in the alphabet. The devious way in which the different letters could be conjured up made Enigma fiendishly difficult for an outsider to crack, even if he had had a machine. The message-sender and the recipient organized the machine in a different way each time that a message was sent. Both followed instructions in an operating manual. Before the war the instructions were changed monthly; toward the end this was done three times a day. The result: any message could be garbled into any one of a possible 10,000,000,000,000,000,000,000 different codes. To crack it would require exceptional ingenuity. Living and working at Bletchley Park were some of the foremost code-breaking brains in the world, including Alan Turing, one of the greatest mathematicians of the time. He invented a 10 -foot-high electromechanical device that could test combinations of coded letters from Enigma at high speed. The device contained more than 30 sets of rotating wheels; each could sift 17,576 combinations at one time. Unknowingly, the Germans themselves helped the British by repeating in their messages certain phrases such as "by order of the Fuehrer," "with reference to", or "commanding officer." Once deciphered in one code, the message offered vital clues to the structure of other codes using the same phrases. Consistently, German operators would also align the three rotating wheels with the same three letters - such obvious combinations as ABC or XYZ - over and over again. This knowledge reduced the number of possibilities the code breakers and to consider. The machine itself also had some limitations. For example, it was incapable of encoding any letter of the alphabet as itself - W, for instance, would never appear in code as W. The British used this knowledge to their advantage. Occasionally they would send an aircraft out to bomb an insignificant target. Experts could then find the name of the place in the encoded Enigma traffic that reported that incident; they search for groups of letters that did not contain any of the letters of the real name in the same position. This was a way into the balance of the code. Soon the British were reading Enigma messages, and Bletchy Park was able to relay enemy bombing plans, U-boat positions and the strength and deployment of ground forces on to the commanders in the field - often with disastrous results for the Germans. Fighter squadrons could concentrate around British cities well before a German attack started; at the same time the settings of the radio-navigation system for German bombers, learned from Enigma traffic could be disrupted. In the Atlantic, convoys carrying vital supplies to Britain were able to avoid a pack of U-boats - as RAF bombers destroyed the vessels supplying the U-boats. In 1940 Enigma revealed that Hitler had abandoned Operation Sea Lion, his planned invasion of Britain; the next year Enigma helped British warships destroy the battleship Bismarck. When British and American commanders discovered Rommel's plans, Allied bombers were able to destroy some of the ships carrying Rommel's troops to North Africa. And in 1944, thanks to Enigma, the Allies knew that the Germans expected the main D-Day invasion forces to land at Calais, not Normandy - and learned exactly what opposition would be waiting for them. Because Allied intelligence was so good, the greatest fear at Bletchley Park was that the Nazi high command would realize that the code had been cracked. Instead, however, the Germans believed they were the victims of an informer or in the U-boat war, the tracking techniques had improved. So closely was the secret of the Enigma guarded that many of the Allied commanders were unaware of the source of the uncannily accurate information they received. Indeed, no intelligence was ever realised that could not plausibly be attributed to some alternate source. The breaking of the Enigma code remained a secret until the late 1970's; even to this day, many details of the equipment and techniques used are not known. But no future historian of World War II can ignore the crucial part the Enigma machine played in the defeat of Germany.

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