In one of the greatest and longest struggles of World War II, the Atlantic Ocean became the key battlefield as U.S. supplies to further the Allied war effort in Europe were shipped to the United Kingdom and, to a lesser extent, onward to the Soviet Union. By the spring of 1943, the Battle of the Atlantic had reached its dramatic climax: The greatest convoy-versus-U-boat battles occurred in March through May.
March is historically viewed as the high point of German submarines’ execution of a guerre de course; U-boats sank 120 merchant ships (nearly 700,000 tons) with only 15 submarine losses. Moreover, the boats sank nearly two-thirds of the ships (half a million tons) while they were in convoy. Convoy—the bedrock of Allied maritime war strategy—was in question. In retrospect, the British Admiralty noted that “the Germans never came so near to disrupting communication between the New World and the Old as in the first twenty days of March 1943.”
However, just eight weeks later, on 24 May 1943, Grand Admiral Karl Dönitz, commander-in-chief of the German Navy, would withdraw the vast majority of his submarines from the North Atlantic, essentially signaling defeat. In a dramatic change of fortune, May concluded with nearly 300,000 merchant tons sunk in exchange for 41 U-boats lost.
U-boats continued to interdict shipping throughout the war to some extent, but many recognize this as the Battle of the Atlantic’s turning point. British synchronization of technical advances, operational reserve, and doctrine were the keys to the Allies’ March-to-May role reversal.
To understand the nature of the campaign, it is necessary to examine briefly the antagonist. The German Unterseeboot (undersea boat, or U-boat) of the period submerged only when necessary for protection or to attack in daytime. In effect, the submarines should have been classified as submersibles. They patrolled on the surface to maximize their visual detection range while maintaining a high speed of advance. The boats normally cruised at 10 knots, but they were capable of 17 knots sustained, as their hulls were designed to travel above the waterline. Underwater, the U-boat had very limited speed and endurance.
Before commanding the German Navy, Admiral Dönitz, as Befehlshaber der Unterseeboote, or BdU (Commander-in-Chief, Submarines), devised Rudeltaktik, or wolf pack tactics. They leveraged the inherent power of concentration of mass at the point of attack. Equally important, the pack formation aided in acquiring initial target cueing data. As Allied convoying evolved during the war, and more merchants were obliged to sail in large groups for safety, the potential for an individual U-boat to visually sight a target decreased. Although the great circle routes were the shortest paths across the Atlantic, thus the most economical and likely to be taken, convoys were not restricted to them, nor would it have been wise to sail them.
To increase the chances of detecting a convoy, wolf packs formed patrol lines that bisected the great circle routes. The distance between each U-boat in a patrol line was set so a convoy could not pass through the line undetected. After initial detection, the spotting submarine would radio updated position data while stalking the convoy, waiting for the wolf pack to mass and attack. Generally, the pack attacked on the surface under cover of darkness and exclusively targeted soft merchants. Before daylight, the U-boats would retire from escort-vessel range and await dusk before again pressing home the attack. The wolf pack coordination and reporting process necessitated volumes of radio transmissions, and BdU directed all actions from ashore. Unknown to Dönitz’s headquarters, the Allies had assistance in avoiding the wolf packs: Ultra.
No World War II operational/strategic discussion is complete without considering the influence of Ultra—code word for British, and later U.S., decrypts of classified German radio and teletype transmissions, most of which were encrypted using Enigma machines. The Poles initially succeeded in breaking some Enigma messages. In Britain, the effort was continued by the cryptanalytic agency located at Bletchley Park, the Government Code and Cypher School (GCCS), led by Alan Turing, the intellectual father of the computer. Decrypt times varied from 5 hours to 11 days. Quicker turnaround times were achieved when using a primer, or “crib,” which was usually a compromised German Short Weather Code book.
For Enigma information to be effective operationally, it generally had to be decrypted within 48 hours because of the relative speeds of the participants at sea. The nominal speed of a slow convoy was 7 knots, while a “fast” convoy averaged 10 knots, but a surfaced U-boat could reach 17 knots. The Admiralty originally used the data defensively to reroute convoys around U-boats. It was not until the end of May 1943 that Ultra intelligence was used for offensive purposes, as the Admiralty was unwilling to rouse enemy suspicion of Enigma’s compromise.
Meanwhile, from February to 10 June 1943, the German Naval Intelligence Service’s cryptological department (Beobachtungsdienst, or B-dienst) could read Allied Naval Cypher No. 3, otherwise known as the convoy cipher. Because of the relative speed advantage of wolf packs versus convoys and the relative speed of breaking the convoy cipher versus Enigma-enciphered messages, U-boats generally enjoyed trump moves; therefore, Ultra was not decisive in the spring of 1943.
Another misconception regarding the Allied change in fortune during March–May 1943 is the extent of U.S. involvement. Certainly, U.S. industrial capacity was vital, but in the spring of 1943, the American war machine was still gaining momentum. Commander-in-Chief, U.S. Fleet, Admiral Ernest J. King and his staff believed in a “hold and build” strategy that would harness the power of American industry and that married well with his “convoy vicinity” concept. Convoys served as bait for U-boats, which escorts and aircraft then could counterattack. With growing Allied surface and air resources, some U-boats surely would be sunk.
Convoy vicinity was an attrition strategy that would not seize the initiative from the Germans. Nevertheless, it remained U.S. doctrine until June 1943, when escort carriers armed with Ultra would take the offensive and destroy the U-boat fleet’s critical at-sea resupply submarines—the Milchkuhe (“Milk Cows”)—at their secret rendezvous points in the Bay of Biscay. Not until 20 May 1943 did King merge the responsibilities of antisubmarine warfare doctrine, technological development, and training under one entity: the Tenth Fleet.
Conference at Casablanca
By the winter of 1942–43, President Franklin D. Roosevelt, convinced of the U-boat threat, had cut back landing craft construction and increased escort vessel production—no insignificant step, based on the need for amphibious lift. In January 1943, Roosevelt, British Prime Minister Winston Churchill, and the Combined Chiefs of Staff met in Casablanca to determine future war strategy.
The participants recognized the U-boat as the primary threat to be overcome before other war efforts could be pursued. The final product of the conference addressed the issue twofold: First, close the North Atlantic air gap—where the majority of the U-boats prowled out of reach of conventional land-based aircraft—with 80 VLR (very long range) B-24 Liberator bombers. At the time, only one Royal Air Force Coastal Command squadron equipped with about 15 VLR Liberators, No. 120 Squadron, patrolled the gap. Second, begin an Allied bombing campaign of U-boat building yards and Biscay bases.
Overall, surface escorts for North Atlantic convoys also were in short supply in the early months in 1943 because of other commitments, including to Arctic convoys, to the Tunisian campaign, and in U.S. waters. The ever-increasing amount of matériel being shipped across the Atlantic only added to the burden and resulted in the convoy cycle being shortened. Twelve ocean-escort groups were formed to accommodate the increased pace: seven British (B1 to B7), four Canadian (C1 to C4), and one American (A3).
On the German side, of 222 front-line U-boats, 45 were in the North Atlantic. For context, BdU started the war with only 56 total submarines, of which 22 were capable of open-ocean operations. In addition to B-dienst’s success rebreaking the convoy cipher in February, the sheer number of U-boats at sea made routing of convoys around wolf packs based on Ultra exceedingly complex. The German Navy also had begun using a new Short Weather Code book that nullified the GCCS’s current crib, resulting in a blackout of Ultra information from 9 to 29 March 1943. The decryption of Enigma messages in a timely manner was a miracle in itself; however, even the short blackout duration was enough to contribute to significant convoy losses.
A running battle involving Convoys HX 229 and SC 122 came to crisis on 16 to 20 March. In short, the route of the faster HX convoy was in the path of the slower SC convoy. Consequently, in a relatively small area in the air gap, more than 41 U-boats attacked more than 90 merchants and their escorts, resulting in one of the largest extended convoy battles of the war. The crisis ended only when VLR aircraft drove the pack down and the convoys came within range of conventional land-based planes, but not before 21 ships of at least 141,000 tons were lost for the cost of one U-boat sunk.
Although high, the 693,389 tons of shipping lost to German submarines in all theaters in March was not a record. But the number of Allied ships sunk in convoy by U-boats that month—72—was an all-time high. Also of concern was that the number of convoy ships sunk in March was more than three times greater than the number of Allied ships sunk while sailing independently (23). Losses in convoy were a grave concern, but changes already were in progress that soon would overcome the U-boats.
Atlantic Convoy Conference
The drama of the HX 229 and SC 122 battle had been preceded by the 1–12 March 1943 Atlantic Convoy Conference. A significant outcome of this meeting of senior U.K., U.S., and Canadian naval and air force commanders in Washington, D.C., was that all North Atlantic convoys above New York City would be the sole responsibility of the British Admiralty and Canadian Navy, while the U.S. Navy focused on mid-Atlantic and Caribbean convoys. King would supply an escort group, A3, to protect convoys in northern waters, but it would be under British operational control. Agreement was reached on a new CHOP (change of operational control) line in the North Atlantic—47 degrees west. The newly formed Canadian Northwest Atlantic Command would be responsible for areas west of the line, and the British Western Approaches Command for the more expansive stretch of ocean east of the line. The changes would take effect 1 April. In addition, greater flexibility was given to operations in general. VLR aircraft were to work to the limit of their endurance regardless of CHOP lines, and escort carrier groups were to begin escorting convoys as soon as possible.
Thus, the convoy conference further reduced U.S. direct involvement in the North Atlantic during mid-March to May 1943. The most significant American contribution during this period would be the activities of the escort carrier group centered around the USS Bogue (CVE-9), which later in March became the first such group in theater. But the British clearly would be setting the tone of Allied operations in the North Atlantic during the spring.
An issue raised at the conference but left unresolved was the lagging pace of U.S. allocations of VLR Liberators to cover the North Atlantic air gap. In February, Britain had only 18 of the aircraft, and no Allied VLR aircraft were operating from bases west of Iceland. The shock of convoy losses in March prompted Roosevelt to ask King about the promised planes, and shortly thereafter, the United States agreed to supply 255 VLR Liberators to close the air gap. Because of logistics and the slow rate of build, by mid-April, only 41 of the aircraft were operational.
Enter Max Horton
In November 1942, Admiral Max Horton, commander of the British Submarine Service, had been appointed to direct the British effort in the North Atlantic as Commander-in-Chief, Western Approaches. At the time, U-boats’ tonnage sunk was near all-time highs. According to Horton’s biographer, the admiral’s doctrine was shaped by the convoys that had experienced “disastrous losses when escorted by a collection of ships strange to one another, untrained as a team and led by an officer inexperienced in convoy protection.” Training as a homogeneous unit and improving antisubmarine technology were his immediate goals.
Horton’s operational strategy is best summarized as “concentration in the decisive area, offensive action, cooperation and economy of force.” The crucial area was the air gap, where he would fight the decisive battle. Admiral Horton modified the convoy vicinity concept offensively by creating “support groups,” which would augment convoys when in distress. The admiral applied an economy of force in two ways. Realizing that the air gap was the most threatening area for convoys, he reduced the convoy escort-force levels by one warship each to form support groups that would operate in the gap. The balance of the support group forces were escorts from the temporarily discontinued Arctic convoys and Home Fleet vessels. Cooperation was in the form of recently arrived reinforcements to land-based and ship-based air assets.
Horton’s support group concept was contrary to the Admiralty’s April 1941 Convoy Instruction: “The safe and timely arrival of the convoy at its destination is the primary object, and nothing releases the Escort Commander of his responsibility in this respect.” As a corollary to this concept, the Admiralty used Ultra in a purely defensive form by rerouting convoys around U-boat concentrations.
Horton advocated for support groups on his first day in charge of Western Approaches Command, but the policy was not approved until after February. His intention was to shock the wolf packs with a coordinated counterattack using the new air assets and support groups. Protecting each convoy crossing the air gap with an independent support group and land-based aircraft had the added benefit of surprise, as attacks on U-boats would come from outside the convoy. At the end of March, Horton had five support groups ready and led by battle-hardened Nelsonian commanders such as Captain F. J. “Johnny” Walker.
All previous efforts came together during the months of April and May. Horton’s strategy of offensive support groups and air support was devastating. Technology was in favor of the Allies. Of note was the performance of high-frequency direction finding (HF/DF, or “Huff-Duff”), which allowed the user to derive a line of bearing from an over-the-horizon radio transmission. By the beginning of 1943, HF/DF was becoming a standard piece of ASW equipment. At least several ships in each escort group were equipped with sets, which resulted in better U-boat position triangulation. The early warning it provided increased a convoy’s ability to reroute itself despite delayed Ultra decrypts. Even with only one bearing, an escort diverted down the line of bearing and forcing a U-boat underwater was almost as good as a mission kill. Because of its relative speed advantage, a convoy could bypass an underwater U-boat.
Aircraft with HF/DF installed or with cueing information supplied by escort forces produced deadly results. Of the 41 U-boats sunk in May, 23 were destroyed as a result of air action. Of these, at least seven were sunk in the Bay of Biscay in a successful continuation of the British Coastal Command’s original air campaign to interdict U-boats en route to their hunting grounds. The high volume of radio traffic U-boats generated to execute wolf-pack tactics proved to be self-defeating. Not learning of Huff-Duff’s existence until later in the war, the Germans believed radar was the reason for the increase in U-boat detections near convoys.
The submarines were equipped with Metox early-warning radar receivers, designed to detect 1.5-meter wavelength radar signals used by the Allies to locate U-boats. But an increasing number of long-range and very long-range Allied aircraft patrolling the bay were outfitted with 10-cm radar sets, whose signals were undetectable.
Notably, surface-vessel radar ranges during daylight periods were generally no better than visual-sighting ranges, as target discrimination was difficult against sea clutter. However, radar was extremely valuable during periods of low visibility and minimal sea-state, as evidenced in early May when 40 U-boats converged on Convoy ONS 5. On one night during this climactic battle fought in fog and on calm seas, 10-cm radar on board Royal Navy warships thwarted all 26 attempts by U-boats to attack the convoy. Although 12 of the convoys’ transports previously had been lost, an unprecedented seven U-boats were sunk, and an additional two were lost when they collided in the fog during the night.
Technology and Doctrine
High-frequency direction finding and radar, especially the advanced 10-cm wavelength sets, enabled the Allies to counter the U-boat’s undetectability. The technological advances proved especially effective when wedded to Horton’s operational strategy. Support groups, when equipped with the latest technology, increased the chances of detecting U-boats by sheer numbers of platforms engaged simultaneously.
The doctrine of offensive convoy support, versus convoy vicinity defense, allowed the British to set the tempo and seize the initiative in the North Atlantic. By mid-May, the Western Approaches commander discontinued convoy evasion in favor of routing certain convoys into U-boat concentrations. As he explained, “The shy tactics of the U-boat demands that the bait should be put right under its nose before it will take risks, and so give us a chance to make kills.” Instead of focusing on the safe and timely arrival of convoys, the British now were prepared to lose two merchant vessels in exchange for every U-boat sunk. The rate proved unacceptable to Grand Admiral Dönitz.
William Scott Chalmers, Max Horton and the Western Approaches: A Biography (London: Hodder and Stoughton, 1957).
Karl Dönitz, Memoirs: Ten Years and Twenty Days (Annapolis, MD: Naval Institute Press, 1990).
Bernard Edwards, Dönitz and the Wolf Packs: U-boats at War (London: Arms and Armour, 1996).
F. H. Hinsley, British Intelligence in the Second World War (Cambridge, UK: Cambridge University Press, 1979).
David Kahn, “Codebreaking and the Battle of the Atlantic,” U.S. Air Force Academy Harmon Memorial Lecture #36 (4 April 1994).
Montgomery C. Meigs, Slide Rules and Submarines (Washington, DC: National Defense University Press, 1990).
Marc Milner, Battle of the Atlantic (Stroud, UK: Tempus Publishing, 2003).
Peter Padfield, War Beneath the Sea (New York: John Wiley & Sons, 1995).
Henry Probert, “Allied Land-Based Anti-Submarine Warfare” in Stephen Howarth and Derek Law, eds., The Battle of the Atlantic (Annapolis, MD: Naval Institute Press, 1994).
Jürgen Rohwer, The Critical Convoy Battles of March 1943 (Annapolis, MD: Naval Institute Press, 1977).
CAPT S. W. Roskill, The War at Sea, 1939–1945, vol. 2 (London: Her Majesty’s Stationery Office, 1956).
John Terraine, The U-Boat Wars: 1916–1945 (New York: G. P. Putnam’s Sons, 1989).
John Winton, Ultra at Sea (London: Leo Cooper, 1988).