In early 1941 at Bletchley Park, civilians lacking military experience above parade-ground level and ignorant of the vocabulary of command - I was one of them- were suddenly empowered to send Enigma-derived signals to GHQ Cairo about Rommel's activities in Egypt. We learned about German plans to attack Crete a fortnight before the invasion was launched. From then onwards we were always busy, increasingly so as D-Day approached; by 1944 there was so much Ultra originating in France, Italy and elsewhere that it required a cool head to keep abreast of developments in all theatres and to switch attention from one to another perhaps several times an hour.
Crete was lost, mainly because there was insufficient force to take advantage of the forewarning. The perfect revenge did not come until two years later. By retreating through Tunisia, Rommel gave advance notice that he would counter-attack with 31,000 men and 135 tanks at a place called Medenine on 6 March. Montgomery dug in 600 anti-tank guns and quickly knocked out 50 German tanks. Unless there is force to back it, intelligence is useless; given sufficient force, intelligence sharpens conquest and can even create the opportunity for it.
Ultra first made its mark in late summer 1942. With Rommel poised for a final thrust on Cairo, Churchill appointed Montgomery to the command of 8 Army. Montgomery drew up a plan of action, divining what Rommel would do and how to foil him. Two days later, Rommel told Hitler exactly the same - in Enigma. The defensive battle of Alam Halfa paved the way for the victory of Alamein. The Montgomery legend was born and Ultra's reputation established.
Ultra's greatest triumph was to reveal Hitler's biggest mistake. By the end of July 1944 the British 21 Army Group was massed round Caen, the Americans had taken Cherbourg and were advancing down the west coast of the peninsula. The Germans tried to cut off their advance on 6 August. On 10 August Hitler personally ordered a renewal of the attempt - thus committing his armour westwards although his main danger was from his rear in the east. Both allies took immediate advantage. Two German armies were surrounded and annihilated in the Falaise pocket. Paris fell on 25 August.
In the euphoria of victory everyone from the Chiefs of Staff down excitedly proclaimed, "Hitler is beaten, the war is almost over." Only Churchill kept his head, saying on 8 September that it was at least as likely that Hitler would still be resisting at the beginning of the New Year as that he would collapse before then. In fact, Arnhem was still to come and the surrender did not follow until May 1945. The sobering lesson was the old one about not counting chickens before they are hatched.
It has been said that Ultra shortened the war by two years. This is pure speculation, however plausible it may seem - even in 1945 new and faster U-boats fitted with Schnorkels to make them undetectable might still have cut the supply-line which fed Britain and fuelled Normandy, had they been developed just a few months earlier. What is certain, however is that from at least late in 1942 Ultra gave such a profound insight into the intentions of the Nazi High Command and the activities of its subordinates that its contribution to victory was nothing less than tremendous.
Ralph Bennett is the author of `Behind the Battle' (Pimlico, pounds 12.50)