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Harry Hinsley closes a counter-factual-historical assessment of the role of Enigma decryption:
The U-boats had delayed the invasion [of France] only by months, till the spring of 1945, other considerations would have come into play. As it was, the invasion of Normandy was carried out on such tight margins in 1944 that it would have been impracticable—or would have failed—without the precise and reliable intelligence provided by Ultra about German strengths and order of battle. Carried out in 1945, [Operation Overlord] would have failed more decisively—or, more likely, these other considerations would have necessitated further delay. Germany's V-weapon offensive against the United Kingdom, and especially against some of the invasion ports along the south coast, would have been in full swing, creating immense destruction throughout southern England. [Germany] would have finished the Atlantic Wall. From early in 1945, as Ultra revealed, [Germany] would have brought into service revolutionary new U-boats and jet and rocket aircraft.... Who can say what different strategies [the Western Allies] would have pursued? Would the [Soviets] meanwhile have defeated Germany, or Germany the Soviets, or would there have been stalemate on the eastern fronts? What would have been decided about the atom bomb? Not even counter-factual historians can answer such questions. They are questions which do not arise, because the war went as it did. But those historians who are concerned only with the war as it was must ask why it went as it did. And they need venture only a reasonable distance beyond the facts to recognize the extent to which the explanation lies in the influence of Ultra.

Any other thoughts on how Ultra shortened the war?

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