Various branches of the German military – the Abwehr, Luftwaffe, Wehrmacht, navy and so on – all used variations of the Enigma machine with their own specific encryption settings. Each of these had to be deciphered independently at Bletchley Park.
There is little doubt that the resulting intelligence – named ‘Ultra’ by the British – made a very considerable, even decisive contribution to the Allied war effort. Its success stories are legion. It was through Ultra, for example, that Rommel’s plans were divined on the eve of Alam Halfa, a battle that spelt the beginning of the end of the Afrika Korps. It was Ultra intelligence that forewarned the Soviets of German intentions at the battle of Kursk, defeat in which robbed the Wehrmacht of its offensive capacity. The cracking of the Italian naval code, meanwhile, facilitated the Allied victory at the battle of Cape Matapan in 1941, which checked Italian ambitions in the Mediterranean.
D-Day itself would scarcely have taken place, never mind succeeded, without the crucial intelligence provided by Bletchley Park. At every turn, it seems, the Allies drew enormous benefit from their ability to read enemy signals, and thereby learn what their opponents had in mind. Taken together, therefore, the various strands of ‘Ultra’ intelligence were clearly of vital importance to the Allied cause, yet any definitive assessment of their comparative merits is very difficult to make.
For argument’s sake, therefore, I will plump for the cracking of the German naval codes in 1941–2, by which the balance in the battle of the Atlantic was decisively shifted in favour of the Allies. Were it not for that achievement, Britain might well have been rendered lame militarily or starved into submission long before D-Day, thereby rendering Ultra’s other successes rather superfluous.
Answered by Roger Moorhouse, author of Killing Hitler (Vintage, 2007).