In late March the first American escort carriers began appearing in the Atlantic; these were small ships that carried twenty-four fighters and navy attack planes and extended air cover across the Atlantic to the convoys. Shipboard direction-finding equipment was now paying off as well; these sets allowed escort destroyers to run down the line of a radio bearing to intercept a U-boat before it got in range of a convoy, increasing by probably 50 percent the number of U-boats sunk by destroyers in February, March, and April according to a study by Rodger Winn’s Operational Intelligence Centre. It was air attacks, however, that now were really beginning to tip the balance. In April and May, fifty-six U-boats were destroyed, thirty-three of those in whole or in part the result of air attacks.44
On May 19, Dönitz, who earlier in the year had been promoted by Hitler to commander-in-chief of the German navy with the rank of Grossadmiral, broadcast to his commanders an extraordinary do-or-die order:
If there is anyone who thinks that fighting convoys is no longer possible, he is a weakling and no real U-boat commander. The Battle of the Atlantic gets harder but it is the decisive campaign of the war. Be aware of your high responsibility and be clear that you must answer for your actions.… Be hard, draw ahead, and attack. I believe in you. C-in-C.45
It took just four more days for reality to sink in even to Dönitz. On May 23 he withdrew his U-boats from the Atlantic convoys and ordered a “temporary shifting of operations to areas less endangered by aircraft,” the South Atlantic and the coasts along Gibraltar, Brazil, and Africa. He noted in his diary that not long before he was sinking 100,000 tons of enemy shipping per U-boat lost; that exchange rate was now down to 10,000 tons per U-boat. “Thus losses in May have reached an intolerable level,” he acknowledged.46
In June, the U.S. Navy’s code breakers in Op-20-G did what Dönitz’s security investigation earlier in the year had failed to, and tracked to the source their growing suspicion that there had to be a leak somewhere in their own code systems. As early as August 1942, Bletchley Park had warned that German radio messages contained information obtained from reading the Allied convoy code. But GC&CS had only one cryptanalyst assigned to checking the security of their own ciphers and the British naval officials in charge of code production were slow to react and convinced themselves that some stopgap changes introduced in December had mostly fixed the problem.47 The American code breakers now carefully matched up decrypted Enigma signals advising the U-boats of “expected convoys” with British and American messages that had been transmitted shortly before in Naval Cypher No. 3. It had required a huge fight to get the security-conscious Admiral King to agree to release copies of the American convoy signals to his own navy’s investigators, but once they finally got hold of them the match was perfect. The Germans’ precise information about convoy movements clearly had come directly from the U.S. Navy’s own signals. On June 10 a new convoy code, Cypher No. 5, was ordered into service. For the rest of the war, the Allied convoy signals remained unbroken.48
The ASWORG scientists also had been denied access to the Enigma secret on King’s orders, but they figured it out themselves—incidentally providing an example of what Dönitz could readily have done had he had an organized scientific staff of his own, which he did not (at least until late in the war, when it was too late to make a critical difference). Morse had assigned Jacinto Steinhardt, a Ph.D. chemist who joined ASWORG in November 1942, to carry out a study of how well Allied radio direction fixes contributed to locating and attacking U-boats. Steinhardt decided to first see how good the data was, and so compared the direction finding reports to the actual locations where U-boats were subsequently located. The DF fixes were unbelievably accurate; in fact, Steinhardt calculated, ten times more accurate than ought to be possible through direction finding alone. Morse and Steinhardt had little trouble deducing the true explanation—that decoded German messages were the real source, which for security reasons were being disguised by the U.S. Navy as DF reports.
At his next meeting with Admiral Low, Morse reported this curious result “with a straight face.” He said:
Admiral Low, also with a straight face, said that was interesting. But the next day he called Steinhardt and me in and disclosed what by that time we had guessed but never mentioned, that our side had broken the German code and that the locations given to us as [DF] readings were in fact the positions reported by the submarine skipper himself to his commander.49
Enigma decrypts throughout the spring were revealing something of potentially even greater significance than the U-boats’ whereabouts: the growing demoralization and even panic of their crews. In his May withdrawal order, Dönitz vowed the battle would soon be rejoined in the North Atlantic, “the enemy’s most sensitive area.” The U-boat arm, he swore, would soon “subdue the enemy by a continual bloodletting which must cause even the strongest body to bleed to death.” But such heroic language was apparently lost on the men under him, now being killed by the thousands a month: Rodger Winn noted in an intelligence report that in April and May, for the first time in the war, U-boat commanders had failed to press home attacks on convoys, even when they were in a favorable position to do so. Enigma decrypts showed that a sharply increasing number of boats were declaring “mechanical difficulties” during their outbound transit of the Bay of Biscay and turning back to port.
Dönitz made the rounds of the bases giving pep talks, but the mood was morose. “No more parties were given to celebrate the start of a campaign now,” recalled one commander. “We just drank a glass of champagne in silence and shook hands, trying not to look each other in the eyes.”50
The battle of the convoys was, in fact, all but over.
IN PRESSING FOR stepped-up air operations in the bay back in March 1943, Blackett had anticipated this development, foreseeing that the withdrawal of the U-boats from the North Atlantic would offer an even more compelling reason to focus the anti-U-boat offensive on the bay; it would also offer an opportunity to deliver the coup de grâce to Dönitz’s force. As long as the U-boats remained engaged in attacking the convoys, there was some truth to King’s insistence that the “convoy area” was the proper focal point for the application of air power: the convoys were the bait that drew the U-boats into the killing zones of escorting aircraft. But, as Blackett argued with remarkable prescience in a March 23 note to the naval staff, that situation would surely not last. At present the use of VLR aircraft to provide air cover to shadowed convoys was undeniably the best way to employ them; measured in terms of both ships saved and U-boats sunk, each flying hour produced ten times greater returns in that mission than when employed patrolling the bay. But the comparison was “misleading,” Blackett cautioned, and certain to change:
… at any time, BDU may find their present offensive against the transAtlantic convoys unduly expensive and decide to switch their effort to other areas. Though this would be an important gain for us, we may still lose very heavily by attacks on other convoy routes, independent and in focal areas. It is clearly impossible for us to be equally strong in potential air cover all over the Atlantic (and Indian Ocean) so that “soft” spots must necessarily exist. We would then be forced to switch also our V.L.R. air effort to the threatened area to supplement the normal air cover available there. This is a difficult operation, requiring a large preparatory ground effort, and leaves the initiative in the hands of the enemy. Thus the giving of air cover to shadowed convoys in the N. Atlantic, while extremely profitable now, is unlikely to become a decisive method of defeating the U-boats, since BDU can disengage at will. The advantage of the Bay offensive is that it acts against all U-boats using the Biscay ports, i.e., the great majority of all operational U-boats. BDU cannot disengage from the Bay offensive at will.51
Blackett attached an analysis that he and Williams had carried out on the number of VLR aircraft needed to thoroughly cover the bay. It was definitely very much fewer than the numbers that would be needed to protect ships all across the Atlantic and Indian oceans on
ce the U-boats shifted their hunting grounds: “certainly less than 300 aircraft, and probably less than 200.” Just forcing the U-boats to stay submerged as much as possible while transiting the bay cut down their operational time at sea significantly. And Blackett noted that even if the U-boats adopted maximum submergence tactics while crossing the bay, they would still have to cross half the distance on the surface, where they were vulnerable to detection by radar and to attack: that was because their submerged speed was so much slower. There was thus a limit to how much the U-boats could ever successfully evade aircraft. Saturating a band about 120 miles wide with constant patrols day and night for twenty-four hours would in theory catch every U-boat passing through that zone during the period.
Even when sightings did not offer a chance to carry out an attack, forcing the U-boats to dive when air patrols passed over produced increasing returns on each additional investment in flying time. Each time a boat dived it had to spend an extra quarter of an hour on the surface recharging its batteries just to replace the energy expended diving and resurfacing; that extra time on the surface in turn increased its chances of being spotted by another air patrol. More flying hours thus increased the number of U-boats sighted per flying hour. In March and April, when Coastal Command aircraft began to be equipped in substantial numbers with 10-centimeter radar, Allied aircraft were already spotting 60 percent of the U-boats that made the transit across the bay—which worked out to one for every 30 flying hours—and attacked 40 percent of them.52
Those numbers shot up in June and July. Blackett and Williams and others in the Admiralty ORS produced a series of studies on the most effective balance of day and night flying, the advantages of regular versus irregular patrols, the best area and proper width of the patrol patterns, the expected German countermeasures and how best to deal with them. From March to August, aircraft sighted 350 U-boats in the bay and attacked 200 of them. In June, the U-boats began transiting the bay in groups hoping to fight off air attacks with massed antiaircraft fire from their deck guns; the Allies countered by sending surface vessels into the bay to add an additional threat the U-boats had to counter.53 At the peak of the bay air offensive, every operational U-boat was being sighted on average nearly twice each month by Allied aircraft. From July 2 to August 3, the Allies destroyed 41 more U-boats, 16 of those by air attacks in the bay.54 The U-boat war, as Admiral Noble had said, was now indeed an air problem: in June through September naval and shore-based aircraft destroyed four times as many U-boats as surface vessels did. So it would remain throughout the rest of the war, with aircraft consistently accounting for one and a half to two times as many kills as surface vessels.55
Churchill would reflect in his memoirs on this turning point of the war in the summer of 1943, when “the decisive battle with the U-boats was now fought and won,” and the fear and caution yet remained:
The Battle of the Atlantic was the dominating factor all through the war. Never for one moment could we forget that everything happening elsewhere, on land, on sea, or in the air, depended ultimately on its outcome, and amid all other cares we viewed its changing fortunes day by day with hope or apprehension.… Vigilance could never be relaxed. Dire crisis might at any moment flash upon the scene with brilliant fortune or glare with mortal tragedy.56
But hope was in the ascendant even as vigilance remained. One incredible fact stood out: for the month of June 1943, the number of merchant ships lost to U-boats was for the first time no greater than the number of U-boats sunk.57
IN JUNE the first of the American four-rotor bombes were completed and running, finally turning the daily U-boat Enigma solution into an almost routine assembly-line process, which would continue to the end of the war as dozens more of the machines were shipped in the ensuing months to Washington from the National Cash Register factory in Dayton.58 Bletchley Park would teleprint bombe menus to Washington; the U.S. Navy WAVES who operated the bombes at the Naval Communications Annex, located in a former girls finishing school that the navy had seized at the start of the war in the northwest of the city, set up the machines; and a few hours later the results were on their way back across the Atlantic, and decoded Enigma messages flowed into the Operational Intelligence Centre at the Admiralty and the “Secret Room” within the submarine tracking center at Main Navy in downtown Washington.59
To extend the time the U-boats could operate at sea and to minimize their repeated passages through the bay, Dönitz had ordered construction of a dedicated fleet of U-boats that carried large fuel tanks—the “milk cows”—that could refuel the operational boats at sea. Signals specifying rendezvous points were radioed up to two weeks ahead of time. Low argued vigorously for putting this intelligence bonanza to use. Knocking out the tankers, he thought, would be a huge leverage point; each one sunk would pay off in multiples of reduced U-boat days at sea for the entire fleet. There was the chance as well of catching several U-boats in one spot.
The Admiralty sought to veto the plan, fearing that it was too risky: since the actual rendezvous were carried out in radio silence, a successful attack would surely put the Germans on notice that the Enigma had been compromised. Low went ahead anyway. In June, July, and August 1943 the U.S. Navy’s escort carriers, now operating in independent task forces, carried out a series of crushing attacks on the tanker rendezvous around the Azores and elsewhere in the Atlantic, sinking 3 of the milk cows and 11 operational U-boats.60 For the first time in the war the number of operational U-boats at sea fell, dropping by almost half from an average of 120 in May to 60 in August. More important, the effectiveness of each boat plummeted: even when they did operate they accomplished almost nothing. For the rest of the war Dönitz’s U-boat fleet barely managed to sink 0.1 merchant vessel per month for each U-boat at sea, down from a peak of forty times that at the height of their prowess. The plots of the Atlantic showing the location of every merchant ship sunk that had been clouds of dark smears covering the Western Approaches of Britain, the east coast of the United States, the ominous “air gap” halfway across the North Atlantic, gave way to a few scattered dots barely noticeable amid a sea of tranquil emptiness: the German submarines, Admiral King announced, had been reduced from a “menace” to a “problem.”61
“Probably the antisubmarine campaign in 1943 was waged under closer scientific control than any other campaign in the history of the British Armed Forces,” Blackett would write later in an appreciation of E. J. Williams.62 It was, as Churchill had warned, a bitter war “of groping and drowning … of ambuscade and stratagem”; but suffusing it all was a war of science. The Allies outfought the Germans at sea: but most of all they outwitted them.
Political Science
ONCE AGAIN a file of U-boats meekly made their way to British ports. Again the sinister silhouettes of an elusive and loathed predator resolved themselves into the contours of a domestic creature, almost pathetic in their helplessness as they chugged along at half speed, wallowing through the waves.
Three weeks before the German surrender on May 8, 1945, the British commander-in-chief, Western Approaches, anticipating the end of five and a half years of gnawing sleepless nights and anxiety-ridden days, sent orders to the fleet specifying procedures to be followed:
On the “Cease Fire” U-Boats will be ordered by the Admiralty:
a. To report position
b. To remain fully surfaced
c. Fly large black or blue flag by day
d. Burn navigation lights by night
e. Jettison ammunition at sea and render all accessible torpedoes safe by removal of pistols. Render mines safe, and remove breech blocks from gun
f. Proceed to harbour or sheltered water for boarding and preliminary inspection
g. To make no signals except in P/L [plain language]
h. To adjust speed to arrive at (f) between sunrise and three hrs. before sunset1
Instructions to Allied ships and aircraft directed that they were not to take any offensive actions against a sighted U-boat except i
f it “is seen to commit a hostile act or shows signs of treachery, for example by diving or failing to comply with the orders.”
The cease-fire went into effect at midnight May 8 Central European Time and nine hours later, just after sunup the next morning in the United Kingdom, the first radio signal from a U-boat reporting its location was picked up by Coastal Command. A Sunderland flying boat was immediately diverted to its reported position, about 100 miles northwest of the Irish coast. A little after noon the aircrew sighted a surfaced U-boat, flying a blue and white flag from the conning tower. The crew was crowded on the deck “waving madly and giving the ‘thumbs up’ sign,” the British plane reported. Over the next week 173 more German submarines surrendered. Defiant to the end, 221 German commanders scuttled their boats rather than allow them to fall into Allied hands. Two made it to Argentina.2
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