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Having just watched The Imitation Game, I am curious about the influence of the cracking of Enigma on the outcome of WWII.
Which battles did the information decoded by Enigma influence? Which battles would likely have been lost without the cracking of the code?
According to "Marching Orders", Enigma had a decisive impact on the (Second) Battle of El Alamein. Montgomery's first attacks were thrown back with heavy loss. Under different circumstances, he (or another commander) might have broken off the attack (as General US Grant did at Cold Harbor, in the Civil War).
But Enigma alerted Montgomery to the fact that Rommel's losses were just as heavy in absolute terms, and twice as heavy in proportional terms (Montgomery outnumbered Rommel by at least two to one), and that Rommel was in "truly terrible shape." So Montgomery continued the assault and destroyed Rommel's army by attrition.
Enigma played a part in later campaigns, in Italy, Normandy, etc., but never as decisively as at El Alamein.
The most obvious single battle that was influenced by ENIGMA may, ironically, be the (early stages of the) Battle of the Bulge. Hitler's paranoia had finally advanced to such state in 1944 that he was convinced the Allies were eavesdropping on his intelligence; so he insisted that all plans drawn up for that attack NOT use the cracked cypher. It is likely that the Allies had become used to never being grossly surprised, leading to a degree of over confidence that cost lives in December 1944.
Note that even though Hitler's conclusion was correct, this is still really just Hitler's paranoia showing though. The use of ENIGMA intelligence by the Allies was so sparing that he had no logical basis for this conclusion. Other than in the North Atlantic, where the battle was truly existential for the British, it was kept as a reserve to prevent exactly the scenario that occurred Dec. 16, 1944 - a massive German surprise attack in a weakly defended sector.
In the Allied favour the key battle/campaign would be that of the merchant marine convoys in the North Atlantic through the dark days of 1941 and 1942. The British never realized that the Germans had broken the merchant marine codes in the mid 1930's, so it was only with the help of ENIGMA intelligence that sufficient convoys got through to keep Britain afloat as it were.
this is still really just Hitler's paranoia Just because you're paranoid doesn't mean they're not after you :-)
The D-Day Landings.
The British Security Service (MI5) were operating a number of double-agents in UK territory, pretending to their German controllers that they were still diligently spying on the UK, but actually sending back misinformation. Because Enigma was cracked, MI5 realised that every single message being sent back to the German intelligence service was from these double-agents; this meant that there was no intelligence reaching Germany except that being generated and controlled by MI5.
Armed with this knowledge, they launched Operation Fortitude to deliberately feed controlled disinformation to Germany via the double-agents, misleading them as to the location of the planned D-Day landings. This was so successful that, even after the landings were well underway in Normandy, 15 German army divisions were kept in the Calais region to repel the supposed 'real' invasion there which was supposed to follow the 'diversionary' Normandy invasion. This was quite possibly crucial in allowing enough time for the invading forces to establish a beachhead strong enough to support the continued assault.
The battle of Medenine, March 6th 1943, was influenced by ENIGMA.
Before this battle started, the situations was as follows:
Now, why had the Germans the use of radio communications? Because phone communications were not that much developed in Tunisia, and French had sabotaged some. => So they used radios and Enigma helped decoding the message
Enigma helped knowing the place of the offensive because Rommel had to tell to dispersed armored units where to meet. This was probably as well a lack of cautiousness from German radio operators.
So Montgomery concentrated antitank guns, minefields and tank reserves and defeat the Germans suffered 50 tank as well as infantry losses.
Without Enigma, the German defeat would have not been that big and losses not that high. Moreover, British tanks would have suffered losses as well and that would have hampered later offensive.
