“As the result of the most strenuous endeavor a few days’ U-boat traffic will be readable in the immediate future and this may lead to better results in the near future.” Admiral Dudley Pound to Admiral Ernest J. King,
13 December 1942
With this signal, the British First Sea Lord informed his U.S. counterpart, the Chief of Naval Operations, of the long-anticipated reacquisition of one of the most valuable sources of naval intelligence exploited during World War II. The German U-boat cipher, produced on the Enigma machine, largely had resisted efforts to break it since its revision in February 1942. Based on their experiences with earlier U-boat ciphers, the British expected these decrypted signals, referred to by the security classification ULTRA, to yield invaluable information on the composition, disposition, and intentions of the U-boat command. Having conveyed the good news, however, Pound felt compelled to add, “You will, I am sure, appreciate the care necessary in making use of this information to prevent suspicion being aroused as to its source. ... It would be a tragedy if we had to start all over again on what would undoubtedly be a still more difficult problem.”1 Pound’s comments reflected the classic tension between exploiting available intelligence and the fear of compromising the source. This debate over the use of ULTRA in the Atlantic was to become a major source of contention between the British and U.S. navies.
The Royal Navy had broken an earlier variant of the German submarine cipher in March 1941. By August 1941 the British typically could read German messages with minimal delay, only to lose the capability early the following year.2 Acutely aware of how much could be lost, the British adopted an extremely conservative attitude toward the exploitation of ULTRA. Decrypts could be used to direct convoys away from known areas of U-boat concentration, but not to hunt the U- boats themselves. While the broad consensus was that convoy rerouting was a necessary and helpful countermeasure, it was extremely difficult to assess its exact effectiveness, and in the absence of hard information it did not provide the same reassurance found in U-boat kills.
These policies, however, had been set prior to the U.S. entry into the war. By the December 1942 breaking of the “Shark” cipher, British and U.S. codebreaking efforts had become a largely cooperative venture, and the new partner did not feel constrained by Old World conservatism. The preceding year had proved an extraordinary trial for U.S. shipping in the Atlantic. November 1942 had been the worst month of the war in terms of worldwide merchant shipping losses, with more than 720,000 tons falling victim to the U-boats. Of these losses, 28 ships had been lost from Atlantic convoys, while another 47 had been lost while steaming independently in the Atlantic. Of the latter, 23 were lost off the Americas. While losses off the U.S. East Coast had declined since the summer months, they had a major impact on U.S. antisubmarine policy.’
It was against this background that the U.S. inclination toward offensive action first appeared. In January 1943, a memo circulated within King’s staff entitled “Offensive Action against U-boats.” The two-page unsigned report outlined the argument for exploitation of ULTRA intelligence and identified a key vulnerability of the German U-boat campaign:
With seven refuelling [sic] submarines the enemy is barely able to provide effective logistic measures for the 100 odd [U-boats] now at sea. The loss of several such refuelling submarines would decrease [U-boat] effectiveness out of all proportion to the number involved. With the information currently at our disposal we are now able to determine up to a week or ten days in advance the exact position of the refuelling area.4
The author acknowledged there was a risk of revealing the Allies’ ability to read the German codes if the tanker U-boats were targeted, but concluded that “the risk continues in any event and can be lessened considerably if [U-boats] in general are attacked instead of going after refuelling subs only.”5
The typical German U-boat displaced 500 to 750 tons, and, unless refueled, was restricted to a 500-to- 600-nautical-mile operating radius from home port. This comparatively short range was insufficient for the style of campaign the U-boat command envisioned, so arrangements for resupply had become integral to German operational planning. After an initial reliance on surface resupply vessels, the Kriegsmarine produced two types of dedicated submarine tankers: one a combined minelayer and refueler, the other a much larger boat designed exclusively for resupply. The first of these specialized craft became operational in April 1942.6 By February 1943, eight such vessels had been completed. The tempo of U-boat operations put terrific demands on the U- tanker fleet even at its height, regularly demanding more than half of each unit’s time be spent on active patrol. During spring 1943, three or four refuelers normally were active at a given time, the others being either in port for servicing or en route to or from their assigned stations.7
The U-tankers were, in fact, highly effective in permitting the U-boat command to extract the maximum possible effort from its limited resources. When the U-tankers were introduced in spring 1942, the average U-boat patrol was 41 days. A single refueling could extend this patrol to 62 days, while two refuelings could yield a patrol of 81 days. At that point, other maintenance and supply constraints required a return to port. By May 1943, some 390 refuelings had been performed by the U-tanker fleet in the Atlantic.8
By spring 1943, the idea of using U.S. submarines against German U-tankers officially was proposed to King by Rear Admiral Francis Low. In an “ultra secret” memorandum dated 14 April 1943, Low proposed that “even two (2) submarines constantly on station in small areas that either we or the British can designate (based on exact knowledge, not on estimates) would be productive.” His primary concern was securing the submarines needed. He suggested they request from “the Admiralty that two of our submarines now under their operational control be assigned this task.”9
On 20 April, King, having embraced Low’s proposal, formally transmitted it to the First Sea Lord. In his message, King made it clear that operations should be controlled directly by the Admiralty or himself with full use of ULTRA intelligence. Further, he suggested that commanding officers of selected submarines be given indoctrination in ULTRA intelligence.10 The British rejected the proposal, expressing their concern that using ULTRA in offensive operations could result in the loss of this valuable source of information. King replied that, while he was equally concerned about security, he believed the U-tankers could be attacked successfully, and “with careful preparations it seems not unlikely that their destruction might be accomplished without trace.”11
A key feature of King’s proposal was the utilization of submarines to attack the U-tankers. The happy image of these key German assets disappearing without a trace seemed to belay most security objections to the use of ULTRA in offensive action. As antisubmarine platforms, however, submarines of the era had many disadvantages. While it was possible the hunter would find his prey vulnerable and tied up to another U-boat, it was equally likely the hunter would find the U-tanker on alert, awaiting the approach of her customer. In the final analysis, a submarine conducting such operations very easily could have found herself in a position of tactical disadvantage. While the submarine idea eventually was abandoned, King remained committed to offensive exploitation of ULTRA intelligence.
A second feature of King’s proposal to the Admiralty was the use of ULTRA intelligence only under the central control of his staff. As the British had pointed out previously, however, the Atlantic Fleet at the time had no central controlling staff for antisubmarine efforts. On 27 April, King informed the Joint Chiefs he intended to establish a unified command to conduct antisubmarine warfare in the Atlantic. Christened the Tenth Fleet, it possessed no permanently assigned forces, but had the authority to take direct tactical control of naval assets belonging to other fleets to conduct antisubmarine operations.12 Already filling the twin roles of Chief of Naval Operations and Commander- in-Chief, U.S. Fleet, and uniquely positioned to exploit available intelligence, King appointed himself commander of the newly created unit.13
King’s thoughts began to shift to the possible use of escort aircraft carriers as offensive platforms. In January 1943, “jeep carriers” were just becoming available in significant numbers, and the Commander-in-Chief, U.S. Atlantic Fleet, Admiral Royal Ingersoll, indicated to King his intention to use escort carriers to cover the mid-Atlantic air gap as soon as they became available. This gap was a large area of the Atlantic Ocean that could not be covered by land-based aircraft.14 King concurred, and ordered in May that “when the size of surface escort groups become adequate for defensive purposes, excess escorts shall be formed into ‘Support Groups’ to be used either to augment escort groups or to operate in areas of submarine and/or convoy concentrations as appropriate.”15
With the intelligence and forces available, the issue shifted from policy to opportunity and execution. The opportunity presented itself in the form of the U-boat “Group Trutz.” While the 26 May message ordering the formation of the group and its patrol area was not read until 7 June, a 24 May message instructing U-boats to begin traveling to a rendezvous 750 nautical miles southwest of the Azores was read almost immediately. With this information in hand, convoy GUS-7A, on the United States to Gibraltar route, was diverted to the south.16 Located in the vicinity of the convoy was an escort group of four World War I-vintage destroyers and the escort carrier Bogue (CVE-9). Based on the information supplied to him, the commanding officer of the Bogue, Captain Giles Short, positioned his forces to support GUS-7A at its closest approach to the Group Trutz operating area. The Bogue group remained in the area over the next several weeks, positioning itself between Trutz and a succession of passing Allied convoys. Between 4 and 6 June, aircraft from the Bogue attacked 5 of the 17 U-boats in the group, sinking one and damaging another. Most significant, the Bogue group prevented any attacks on Allied merchant shipping.17
On 8 June, the U-boat command sent a message (deciphered on 11 June) indicating that the U-118 was operating in the area of Trutz. A converted minelayer and fueler, the U-118 was exactly the kind of target King was looking for.18 Midday on 12 June, aircraft from Bogue found the U-118 on the surface and, after a 20-minute engagement, sank her.19
The U-118 thus had the distinction of becoming the first U-tanker sunk by Allied action. The impact on U-boat operations was felt immediately. Without her, the U-boat command was forced to cut short some patrols and delay other boats from reaching their target areas. The U-488, another U-tanker, was forced to conduct 22 refuelings. The steady flow of ULTRA intelligence revealed these disruptions, and confirmed the decision to attack the U-tankers.20
The U-l 18 and Group Trutz were the first victims of U.S. offensive antisubmarine action, but they were by no means the last. Vigorous offensive antisubmarine action centered around jeep carriers and their escort groups continued through the following year. King concluded that more specific intelligence would improve the efforts of the escort groups. On 1 July, he wrote to Ingersoll: “Tenth Fleet will transmit as occasion demands . . . more positive and guiding information in the expectation that this will enable those concerned to more quickly alter dispositions to conform to changed conditions which present the opportunity for offensive action.”21
These intelligence estimates, based predominantly on disguised ULTRA intelligence, became a well-respected source of guidance for escort group commanders. Interviewed after the war, Rear Admiral Daniel Gallery recounted his experiences of late 1943 and early 1944 when he commanded the Guadalcanal (CVE-60) escort group:
I treated this . . . daily estimate as Bible truth every day and we based our operations on it completely. One reason why I did was that the very first thing that happened on this first cruise was we got a special message . . . from the Tenth Fleet saying: ‘There is going to be a refueling rendezvous. . . .’ I laid about a hundred miles from that point until about four in the afternoon. Then we launched eight torpedo planes to search [the] area. And right at sunset we . . . blasted the hell out of the refueler and the guy alongside of him. . . . After that experience ... I believed everything that I heard [from Commander-in-Chief, U.S. Fleet] from then on.22
Reflecting on the effect of U.S. escort groups on his U- boat campaign, Admiral Karl Donitz commented, “The U- boat losses, which previously had been 13 percent of all the boats at sea, rose rapidly to 30 to 50 percent. . . . These losses were suffered not only in convoy attacks, but everywhere at sea. There was no part of the Atlantic where the boats were safe.”23
A postwar Navy assessment indicated that of the 489 U-boats sunk by Allied action, after January 1943 U.S. Navy forces sank about 63 with direct aid of ULTRA information, plus some 30 more with the indirect aid of ULTRA.24 Even more important, these carefully directed offensive actions claimed an extraordinarily high number of German U-tankers. Of the 18 Germany produced, 17 were destroyed by July 1944.
Despite this felicitous result, the exploitation of German submarine signals remained a source of contention between the British and Americans. In spring 1944, King characterized the ongoing controversy between the two navies to Admiral Harold Stark, Commander-in-Chief, U.S. Naval Forces Europe:
I assume you are aware of the fact that during the past year there has been a limited exchange of signals between the First Sea Lord and myself as to security matters relating to special U-Boat intelligence. In general these have taken the form of British expressions of alarm regarding our manner of using such intelligence operationally tending towards its compromise, and my replies defending our position.
To give you an idea of our own security measures, thirteen officers in my Headquarters have knowledge of such intelligence and of these only five work with it regularly. None of the U.S. operational commanders in the Atlantic receives such intelligence except indirectly in operational signals.25
Of course, “indirectly in operational signals” covered what the British considered a host of indiscretions. King’s stance, as ever, was clear. The use of ULTRA in the Battle of the Atlantic had been and would remain an integral part of U.S. offensive operations.
The decision to use ULTRA intelligence offensively, over British objections, was in many respects the natural side effect of the increasingly intimate cryptanalytic cooperation between the two nations. King’s decision was based on a number of factors: the vulnerability of the German U-tanker fleet, the rate of losses among Allied merchant ships, and the availability of escort carrier groups as antisubmarine assets. Fundamentally, however, it was the decision of a fighting admiral whose instinct was that intelligence was a tool to be used. British and U.S. intelligence cooperation, born out of need and affinity, was a key element of victory during World War II.
1. Admiral Pound to Admiral King, 13 December 1942, quoted in F. H. Hinsley, et al, British Intelligence in the Second World War: Its Influence on Strategy and Operations, 5 vols. (New York: Cambridge University Press, 1979), vol. 2, p. 233.
2. John Winton, Ultra at Sea: How Breaking the Nazi Code Affected Allied Naval Strategy during World War II (New York: William Morrow, 1988), pp. 22-24.
3. J. David Brown, “The Battle of the Atlantic. 1941-1943: Peaks and Troughs,” in Timothy J. Runyan and Jan M. Copes, eds, To Die Gallantly: The Battle of the Atlantic (Boulder: Westview Press, 1994), p. 149.
4. This document, while undated, is filed chronologically immediately after a similar memo dated 25 January 1943. “Offensive Action Against U-Boats,” no date, COMINCH File of Memoranda Concerning U-boat Tracking Room Operations, 2 January 1943- 6 June 1945, SRMN-032, Record* of the National Security Agency/Central Security Service, National Archives II, College Park, Maryland, Record Group 457 (hereafter cited as NSA/CSS, RG 457).
5. “Offensive Action Against U-Boats,” NSA/CSS, RG 457.
6. Top Secret Studies on U.S. Communication Intelligence during World War II, Part 2: The European Theater, SRH-368 ‘‘Evaluation of the Role of Decryption Intelligence in the Operational Phase of the Battle of the Atlantic,” (hereafter SRH-368), pp. 53-54, NSA/CSS, RG 457.
7. Top Secret Studies on U.S. Communication Intelligence during World War II, Part 2: The European Theater, SRH-008, “Battle of the Atlantic, Vol. II, U-boat Operations,” (hereafter SRH-008), Reel 1, pp. 118-20, NSA/CSS, RG 457.
8. David Kahn, Seizing the Enigma: The Race to Break the German U-boat Codes, 1939-1943 (Boston: Houghton Mifflin, 1991), p. 265.
9. Admiral Low to Admiral King, 14 April 1943, COMINCH File of Memoranda Concerning U-boat Tracking Room Operations, 2 January 1943-6 June 1945, SRMN-032, NSA/CSS, RG 457.
10. Admiral King to Admiral Pound, F-353 Serial 91, 20 April 1943, United States Navy Submarine Warfare Message Reports Admiralty to COMINCH, 1 January 1943-31 March 1943, Serials 171-382, SRH-236, Part I, NSA/CSS, RG 457.
11. Admiral King to Admiral Pound, F-353 Serial 98, 27 April 1943, NSA/CSS, RG 457.
12. Ladislas Farago, Tenth Fleet (New York: Richardson and Steirman. 1986). p. 163.
13. Chief of Naval Operations, Record Group 38, Admiral King to the Joint Chiefs of Staff, 27 April 1943, United States Navy Antisubmarine Organization Correspondence, Tenth Fleet Organization File, National Archives.
14. Admiral Ingersoll to Admiral King, 25 January 1943, quoted in Samuel Eliot Morison, History of United States Naval Operations in World War II, 15 vols. (Boston: Little, Brown, 1956), vol. 10, p. 109. Much of the gap disappeared after British aircraft began flying out of the Azores in October 1943.
15. Chief of Naval Operations, Record Group 38, Admiral King to distribution, 19 May 1943, United States Navy Antisubmarine Organization Correspondence, Escorts File, National Archives.
16. SRH-008, p. 105; SRH-368, p. 46.
17. Morison, History of United States Naval Operations in World War II, vol. 10, pp. 110-16.
18. SRH-368, p. 48.
19. Morison, History of United States Naval Operations in World War II, vol. 10, pp. 113-14.
20. Kahn, Seizing the Enigma, p. 267.
21. Chief of Naval Operations, Record Group 38, Admiral King to Admiral Igersoll, 1 July 1943, United States Navy Antisubmarine Organization Correspondence, Escorts File, National Archives.
22. Rear Admiral Daniel V. Gallery Jr., “U-Boat War from Iceland to Murmansk and the Coasts of Africa,” in John T. Mason Jr., ed., The Atlantic War Remembered: An Oral History Collection (Annapolis: Naval Institute Press, 1990), pp. 122-23.
23. Admiral Karl Donitz, The Conduct of the War at Sea (Washington, DC: Division of Naval Intelligence, 1946), p. 23.
24. Top Secret Studies on U.S. Communication Intelligence during World War II. Part 2: The European Theater, SRH-009 “Battle of the Atlantic, Vol. I, Allied Communications Intelligence,” Reel 1, p. 30, NSA/CSS, RG 457.
25. Admiral King to Admiral Stark, no date (by file and references, April 1944), COMINCH File of Memoranda Concerning U-boat Tracking Room Operations, 2 January 1943-6 June 1945, SRMN-032, NSA/CSS, RG 457.