Seizing the Enigma: The Race to Break the German U-Boat Codes, 1933-1945

by Kahn, David

  Sallenger radioed the Card, 82 miles away, and maneuvered to attack. Selecting the U-boat nearest him, which was slightly behind the other, he approached from the port quarter at 220 knots, out of the sun. “This is it!” he told his crew. The U-66, spotting him, shoved the throttles of both diesels to full speed ahead. When Sallenger was about 400 yards from the subs, both opened fire with their 20-millimeter guns. These filled the sky with white puffs, but Sallenger bored in and, from about 125 feet, dropped two depth charges, set to explode at 25 feet. They straddled the U-117. Three seconds later the explosions raised two columns of water on the starboard side, one about 10 feet out, the other some 20 feet out, cutting the refueling hose. Sallenger banked to the left and climbed. The submarine spurted flame from its stern, and dense gray smoke rolled out. The TBF’s turret gunner, Ammunition Mate Third Class James H. O’Hagan, Jr., sprayed the deck with his .50-caliber machine gun, then concentrated his fire around the machine guns on the conning tower. He saw about twenty men. The radioman took pictures, which would be used to improve tactics.

  The U-boat began maneuvering erratically, as if her steering apparatus had been damaged. She started to trail a heavy oil slick. The U-66 was following her, seemingly trying to help. After about fifteen minutes, the undamaged U-66 started to submerge, apparently in an attempt to save at least herself. While she was thus vulnerable, Sallenger, who had been watching from 6,500 feet, dove to attack. As he flew along her track at 130 knots in level flight, 200 feet up, the U-117 threw intense antiaircraft fire at him. O’Hagan fired back. Sallenger dropped his acoustic torpedo on the last seen course of the submerged U-boat, 150 yards ahead of the diving swirl and 50 yards to starboard some forty seconds after she disappeared. Sun glare prevented him or his crew from observing any results. But the U-66 escaped.

  His armament exhausted, Sallenger soared to 6,400 feet to vector in the other planes. As he circled, the damaged U-tanker tried to dive. For a moment, Sallenger thought she was gone, but she surfaced almost immediately. At 10:33 A.M., twenty minutes after his second attack, two Avengers and two Wildcats arrived from the Card. On command, one of the fighters made a strafing run. He fired a test burst from 2 miles away, but the bullets fell short, and he held his fire until he was in range. During his run, gun flashes from the 20-millimeters at the base of the conning tower winked at him, and he concentrated his fire on this area, though he saw no gunners; apparently they were well protected. He swooped around and attacked from the other side. But the U-boat continued her heavy antiaircraft fire, which forced the lead Avenger, flown by Lieutenant Charles R. Stapler, to weave as it bored in. In a shallow dive, Stapler released two depth charges at 185 feet. They fell close aboard the port side just ahead of the conning tower, and the explosion drenched the submarine. As Stapler pulled up, his gunner strafed the vessel. The first fighter again attacked, and so did the second, just before the second Avenger, coming from the U-boat’s stern, dropped its two depth charges 20 to 25 feet from the submarine on her starboard quarter. Spray covered her. The fighters zoomed down to strafe some more, finally silencing the antiaircraft fire.

  As the two Avengers circled, the crippled U-117 turned to starboard, apparently trying to dive but instead only mushing down, stern first. Then she did go under, and the Avengers turned to attack with their acoustic torpedoes. But they pulled up when the bow and conning tower broke water and the submarine, now barely moving, struggled to surface. Quantities of oil leaked from her. After five minutes, she lost the fight. She began to settle. Her stern went down, her bow rose slightly; the conning tower slipped under, then the bow, and she was gone. Now the Avengers could use their acoustic torpedoes. Stapler dropped his 200 feet ahead of the oil slick and 100 feet to starboard of the U-boat’s last track. Ten seconds later, the other Avenger dropped its 400 feet ahead and to port of where the pilot had last seen the submarine. Some distance away, the crew of the U-66, still submerged, heard detonations, some sharp, some muffled.

  The Avengers circled. A patch of oil 200 feet in diameter where the submarine had last been seen seemed to grow. The radioman of the second Avenger reported seeing a shock wave in the water forward and to starboard of the same point. The U-66 heard crackling noises, and finally sounds that the crew interpreted as those of a boat sinking. The airmen saw a very light blue area that seemed to be caused by small bubbles aerating the water. This persisted for many minutes. Nothing else was seen. At 11:26 the four planes were recalled to the Card. They were relieved by three Avengers which, however, saw neither submarine. Though Isbell claimed that one submarine had been “definitely sunk” and the other “probably sunk,” he was only half right. The U-66 had escaped. But the U-117 had made her last dive. She had gone down about 17 miles north and 40 miles west of where the August 6 U-boat situation report had told Isbell that “probably refueler” would be found. In the vast wastes of the ocean, that was practically pinpointing the target.

  Reporting the episode early Sunday morning, the U-66 did not tell U-Boat Command about the detonations she had heard. U-Boat Command, assuming that the U-117 had survived, gave both submarines a new rendezvous for noon. Later the command observed that with the loss of another milch cow, the last fuel reserve for boats coming from the south had been exhausted, and all fourteen had to refuel now from the U-117. But when the U-66 reported on Wednesday that it had waited two days in vain for the U-117, and when the tanker failed to respond to orders to report, the command concluded that she had been lost during the attack. The critical supply situation forced U-Boat Command into complicated maneuvers: some combat submarines had to give fuel to other boats, then return home using fuel as sparingly as possible. The U-66 made it. The loss of the tankers, Dönitz complained, forced him to end operations in the mid-Atlantic earlier than planned.

  Between June and August, American carrier planes, aided by ULTRA, sank five milch cows and reserve tankers. The British lost all reservations about using Enigma intelligence in these operations. On October 2 the Admiralty asked the U.S. Navy whether it could send a task force against a refueling to take place north of the Azores; Navy planes found four U-boats on the surface and sank the milch cow U-460. A similar request less than a week later ended in the sinking of the combat boat U-220. By the end of October, of the ten milch cows that Dönitz had had in service in the spring only one remained. The effect on U-boat operations was severe. Because resupply by U-tankers was so dangerous, Dönitz avoided it, compelling his U-boats to break off their operations correspondingly early and destroying his hopes for a formidable offensive in distant waters, far from Allied air cover. In November he abandoned the convoy routes as a theater of operations.

  But he returned to the fray the following month. When a patrol line failed to find any ships, he broke it up into subgroups of three boats each in the hope that they would spot targets. It didn’t work. Between mid-December 1943 and the middle of January, they sighted not one of the ten convoys that sailed close to them, and they sank only one merchant ship. At the end of February, Dönitz formed what would be the last wolfpack worthy of the name. PRUSSIA’s sixteen submarines sank two small British warships—at a cost of seven U-boats. On March 22 Dönitz ordered another withdrawal. In the first three months of 1944, his U-boats sank only 3 merchantmen in convoy out of 3,360—at a cost of thirty-six submarines. He persisted with his “wonder weapons”—the acoustic torpedo and the snorkel, a valved tube to the surface that enabled a submarine to run on its diesels while under water, increasing its submerged speed and range. But he concentrated now on sinking shipping around the British Isles for the expected invasion of western Europe.

  ULTRA had little effect on this. The few U-boats dotting the Atlantic posed little threat. The vast convoys, sometimes of hundreds of ships stretching from horizon to horizon, proceeded majestically across the broad expanse of the Atlantic, guarded by sea and by air, bringing the men and materiel that would drive a stake through the heart of the wickedest regime the world had ever seen. With the help of ULTRA
, the Battle of the Atlantic had been won.

  23

  THE RECKONING

  ULTRA WAS THE GREATEST SECRET OF WORLD WAR II AFTER THE atom bomb. With the exception of knowledge about that weapon and the probable exception of the time and place of major operations, such as the Normandy invasion, no information was held more tightly. Churchill’s anxiety about the secrecy of ULTRA was constant; rules in all of the armed forces forbade any action to be taken on the basis of Enigma intercepts unless some cover, such as air reconnaissance, was provided. The security implies ULTRA’s significance. ULTRA furnished intelligence better than any in the whole long history of humankind. It was more precise, more trustworthy, more voluminous, more continuous, longer lasting, and available faster, at a higher level, and from more commands than any other form of intelligence—spies or scouts or aerial reconnaissance or prisoner interrogations. It thus fulfilled better than ever before intelligence’s ultimate purposes, one in the psychological component of war, one in the physical. It improved command, and it magnified strength.

  It improved command by reducing much of the uncertainty surrounding the enemy. As one scholar has written, “ULTRA created in senior staffs and at the political summit a state of mind which transformed the taking of decisions. To feel that you know your enemy is a vastly comforting feeling. It grows imperceptibly over time if you regularly and intimately observe his thoughts and ways and habits and actions. Knowledge of this kind makes your own planning less tentative and more assured, less harrowing and more buoyant.” This benefit of Enigma solutions was intangible but real.

  ULTRA magnified strength in the sea war in several ways. It enabled the Allies to steer the escort-carrier hunter-killer groups toward their prey instead of having to search a large area for them. In sixty days in July and August 1943, when a daily average of fourteen U-boats dotted an area the size of the United States west of the Mississippi, those task forces made forty attacks, sinking thirteen submarines. Since with each attack the task force had found a few U-boats in an oceanic waste the size of Texas, it was doing work that without ULTRA would have required many more task forces. Likewise, a U-boat whose location had been revealed by ULTRA within the previous five days was three times more likely to be sunk than one not so compromised. Thus did ULTRA focus the anti-U-boat efforts and so greatly increase their efficiency.

  Defensively, ULTRA magnified strength relative to the enemy by depriving him of his powers. Steering a convoy around a wolfpack meant that the U-boats could not attack it, thus in effect adding convoy escorts and retaining ships that it would otherwise have lost. Though the value of ULTRA as convoy defense cannot be quantified as precisely as with the offensive operations, some conclusions can be drawn.

  When naval ULTRA was current, U-boats contacted only two-thirds as many convoys as during a blacked-out period. And the rate of sinking of merchant ships in an operational area when ULTRA was current declined to one-sixth of that during a blacked-out period. A comparison on a different basis between ULTRA and non-ULTRA periods concluded that Enigma solutions saved between 1.5 and 2 million tons of shipping in the last half of 1941 and more than 650,000 tons in the first five months of 1943.

  So, did ULTRA win the war?

  Some writers claim that it did. But even as hyperbole this is nonsense. The Allies would have won without it—though at a much greater cost in men and materiel. Some historians argue that “Without ULTRA … the Allies could not have won the Battle of the Atlantic.” This too exaggerates. So does the view that ULTRA stands “at the top” of the factors that influenced the outcome of the Atlantic battle. The most important factor was the construction of an unbelievable number of vessels by American shipyards—so many so fast that even the total effort of all Dönitz’s U-boats was doomed to ineffectuality. Also more important than ULTRA was air cover, which drove the U-boats under water and thereby slowed them so that they could not keep up with the convoys.

  What effect, then, did ULTRA have? Can it at least be estimated how many months of war the solving of the naval Enigma saved?

  Any answer must be hypothetical, and similar calculations could be made about any wartime activity. Nevertheless, it is illuminating to suggest a figure. Without the shipping saved by ULTRA, forces would have been withdrawn from the Pacific to attempt to keep to the timetables for the invasions of Sicily and Italy and, above all, of Normandy. Calculations of ship production and of logistic problems suggest that these invasions would have been delayed by about three months. In particular, the great assault on Normandy might have taken place, not in June 1944, but in the fall, or possibly not until the spring of 1945. During this delay, Hitler’s V-weapons would have caused far greater devastation. The additional submarines that would have come into service would have made crossing the Atlantic and supplying the Soviet Union even more costly in ships and men. The Allied offensives would have come later and perhaps less strongly. The war in Europe might have been prolonged for one year, and because of the withdrawal of forces and supplies from the Pacific to the European theater, the entire conflict might not have ended until 1947. So, taken in isolation, it may be concluded that ULTRA saved the world two years of war, billions of dollars, and millions of lives.

  But events do not occur in isolation. Even if the codebreakers of Hut 8 and OP-20-G had been totally ineffective, even if the war had been prolonged three months or even more because of their inability, something entirely external to them would have taken control of events: the atom bomb. If Germany had continued fighting into the summer of 1945, the first nuclear weapon would probably have exploded not over Hiroshima but over Berlin. And the war would have ended then, no matter what the codebreakers had done, or had not.

  To anyone who looks back at the German navy’s use of the Enigma machine, one question screams out: if the Germans feared that the Enigma was being solved, why didn’t they change to another cipher machine? The answer, upon reflection, is simple. They didn’t have another machine. Should they have prepared one? With hindsight, one can say yes. But several factors stopped them. The rotor principle offered the most secure practical cipher system then known. No other mechanism matched it. The Germans could have adopted a machine similar to the rotor devices of Britain and America, which used five or even ten rotors at a time, and which stepped them in a far more complex motion than the Enigma’s odometer-like regularity. They failed to adopt such a machine for two reasons.

  First, they did not face the reality that Enigma could be broken. The Enigma was an excellent machine, and it was embedded in an excellent web of safeguards against loss, error, and cryptanalysis. The German communications security specialists saw no way that it could be broken. Though they certainly realized that cribs could be used to test for keys, they believed that the vast number of keys defeated this method in practice. They failed to imagine that scores of speedy, brute-force codebreaking machines might be used; their own few cryptanalytic mechanisms were much more primitive. Though the chief judge of the machine’s invulnerability, cryptanalyst Wilhelm Tranow, was not biased in its favor (he had once even urged abandoning the Enigma in favor of a codebook and superencipherment), the officers who did support it would have found it difficult to admit to themselves, to Dönitz, and to Hitler that the system was not invulnerable, as they had repeatedly said it was, and that a new one would have to be created. Finally, because the Germans never had irrefutable evidence of the enemy’s success in cryptanalysis, they never had to concede that the Enigma was broken. That documents were taken from the weather ships never crossed their minds. With the modifications made to accommodate the growing traffic, they viewed the machine as secure. A new machine was not necessary. Indeed, during the entire war, the Germans never even changed the wiring of the Enigma rotors.

  The second reason for not abandoning the Enigma in favor of a new machine was that they had invested too much in the older one. They had bought many machines, distributed them widely, and trained many men in their use. To invent a new system, design a me
chanism for it, test a breadboard model, produce thousands of copies, get them to ships (some of them on long patrols), and to shore stations, teach men to use them, and then put them all into service simultaneously, with the inevitable blunders that would call down the wrath of fighting admirals and generals—this was unthinkable.

  Thus the Germans stayed with the Enigma. But explaining why they did so does not tell why the Allies proved superior to the Germans in codebreaking.

  Perhaps the most important reason was that the Allies, as Poland was before the war, were on the defensive at first. And the defense requires intelligence. Clausewitz defined the characteristic feature of defense as “awaiting the blow.” An army awaits a blow only if it believes that a blow is planned, and such a belief exists only through information about the enemy. The offense, on the other hand, is “complete in itself,” Clausewitz said. An attacking army need not even know where the enemy force is: it can march about, imposing its will, until it meets its foe. An aggressor nation will put more of its energy into men, tanks, ships, planes, and guns and less into intelligence, one form of which is codebreaking.

  A nation that believes it will be attacked will learn what it can about the enemy’s intention. Thus Poland’s fear in the 1920s of German revanche, touching Poland’s very existence as a state, spurred her efforts to solve the Enigma. She established courses in cryptology. Rejewski, perhaps driven by the same patriotic concern, exerted the extra effort that produced his magnificent solution. This is what the Pole Langer meant when, seeking to ease the embarrassment of Bertrand, who had to admit that France had not been able to crack Enigma, he said courteously, “You don’t have the same motivation as we do.”