WI the British code breakers at Bletchley Park hadn't managed to break the Nazi Enigma codes (or U110 hadn't been captured with its Enigma rotors intact, or the 2500 valve Mark II Colossus computer had failed) and Ultra had failed to reveal the U Boat Wolf Pack routes in the critical summer 1941, Rommel's plans, Kursk Offensive info handed to Stalin, D Day preparations etc, could it literally have cost the war, or is Ultra's role overplayed?
First - Enigma was not broken by "the British... at Bletchley Park". Enigma had been cracked in the 1930s by Polish analysts. German changes had defeated the Poles by 1939, but the Poles saw how to overcome those changes - they simply did not have the resources.
So in July 1939, the Poles gave all their findings to Britain
and France. (It was a French spy that had supplied them with the German operating manuals for Enigma and other documents that enabled the initial break.)
Then after the fall of Poland, the Polish analysts escaped to France, where they resumed work under French control, in partnership with the British at Bletchley Park. The break in early 1940 was a joint effort, and both groups deciphered thousands of messages before the French capitulation in June.
Now here's where it gets really weird: the official French intelligence service adhered to Vichy France. However, they arranged for the Poles to resume work at a location in unoccupied southern France. This work continued all the way up to November 1942, when the Germans marched in after TORCH. IOW, the
Vichy French government had the Enigma secret. I'm pretty sure that the spooks never told Petain or Laval about it, though.
But... WI after Mers-el-Kebir, some anglophobic spook leaks the whole thing to Germany. The Germans don't believe it, but they follow up by raiding the Polish center in southern France (illegal under the capitulations, but that's what Skorzeny is for). And ULTRA dies young.
Now, what are the effects?
Considerable at every stage. Some people think that ULTRA was not important because it rarely provided operational intelligence which could be used immediately. What they overlook is that ULTRA provided Allied commanders with an absolutely authoritative picture of German strengths, weaknesses, and expectations.
When one reviews some of the contemporary analysis, it is often shocking to see how mistaken it was. For instance, in 1942, the former U.S. attaché in Romania returned to the U.S. and reported on conditions in Europe. He was certain the Germans had 10,000 or more aircraft and a huge airborne force available to invade Britain.
The British, of course, knew better - much better. They could be
certain that there were no large German forces that they didn't know about. Indeed, they could be
certain about the strengths of German forces on every front and in every branch: they were reading the Germans' own strength returns.
This allowed them to lay out their own plans with near-perfect confidence, and to anticipate most German moves.
ULTRA was helpful, if not decisive, in the Battle of Britain. It provided daily status updates on the German air fleets. Bletchley Park continually read reams of Luftwaffe traffic, which was carefully indexed and analyzed at Bletchley Park. It was said that by 1942, the men who knew the most about the Luftwaffe in all its aspects were in Britain.
ULTRA intercepts were key to the "Battle of the Beams" during the Blitz.
ULTRA in 1941 allowed the Allies to route convoys around U-boat patrols. This ended the first (and most dangerous) success of the U-boat wolfpacks, saving about 2 million tons of shipping.
ULTRA was a major asssist in the "Double-Cross" System: the British controllers could read what the Germans were saying to each other about the double agents.
ULTRA enabled Allied air and naval forces to intercept many supply ships between Italy and Africa.
In 1942, the Allies lost U-boat Enigma, and suffered another period of horrendous losses. But they were able to stay in the game, barely, with naval Enigma from coastal and patrol forces, which escorted U-boats out to and in from sea, and from the U-boat training command.
At the end of 1942, the Allies regained U-boat Enigma, which allowed them both to evade the U-boats at sea, and to find and kill them. By mid-1943, the U-boats were broken - but a mere two weeks of Enigma blindness in March cost over 300,000 tons sunk.
So - remove ULTRA, starting in August 1940.
First effect is during the Battle of Britain. Dowding's policy of reserving fighter strength to ensure that every raid was opposed depended in part on knowing how many planes the Germans had available each day. Without that information, he would have to use guesswork, and would make a lot more mistakes. The Germans would never gain complete control of the air, but they would do a lot more damage to the RAF.
Next is the Battle of the Beams. R. V. Jones figured out that the Germans were using radio beams to direct their bombers in part by reading Enigma traffic that included beam settings.
If the British don't crack naval Enigma... Britain loses a lot of shipping in late 1941, and will have to give up any offensives in the Middle East after July (no CRUSADER). 1942 is even worse than OTL and the losses continue into 1943. This endangers OVERLORD.
The Double-Cross system won't break down - the Germans were just two gullible. But there will be mistakes and failures. The British may not pick up GARBO - they learned of him from ULTRA intercepts. The complete success of BODYGUARD and FORTITUDE in deceiving the Germans won't happen.
Bottom line - much heavier Allied losses in a longer war.