On 22 March 1943, the battered ships of eastbound Convoys SC122 and HX229 reached the entrance of the North Channel, and their sailors breathed a collective sigh of relief. In a running battle across the North Atlantic sea lanes, 44 of Germany’s U-boats, under the leadership of Grossadmiral Karl Dönitz, savagely mauled both convoys within the deadly Greenland “air gap” and sank more than 21 merchant vessels for a loss of 140,842 tons. While the survivors of both convoys sailed toward the relative safety of British ports, the Allied logistical chain between North America and Great Britain teetered on the edge of collapse as the U-boats exacted a heavy price worldwide; in March alone, 108 Allied vessels—for a total of 627,377 tons—were sunk. As one somber staff officer at the British Admiralty observed, “The Germans never came so near to disrupting communication between the New World and the Old as in the first 20 days of March 1943.” The German defeat at Stalingrad and the rapid encirclement of the remnants of Generalfeldmarschall Erwin Rommel’s Afrika Korps in Tunisia in early 1943 indicated a shift in the tide of battle toward the Allies, yet ultimate victory hinged on the ability of the British and American navies to transport men, material, and arms across the Atlantic Ocean.
The longest battle of the war—lasting from the outbreak of hostilities in September 1939 until the German surrender in May 1945—the Battle of the Atlantic, reached its apex in May 1943. The principal aim for both sides revolved over control of the transatlantic sea lines of communication which pitted Allied ships and aircraft against elusive submarines in a complex and rapid competition for advantage. The success of the Allied convoy system was of the utmost strategic importance since control over the Atlantic Ocean served both as England’s lifeline, and as the main prerequisite for any Anglo-American offensive into Europe. The historian Richard Overy, author of Why the Allies Won, correctly concludes that Allied sea power “was the only means by which they could bring other kinds of military force to bear on the enemy, and the only means by which they could fight a genuinely global war.” Thus, the protection of maritime trade routes in the Atlantic, not the destruction of Germany’s battle fleet, became the principal strategic objective for the Allies in 1943. Fought across the northern trade routes between April and May 1943, the battle over Convoy ONS-5 represented Germany’s final attempt to rupture the transatlantic lifeline through a guerre de course—commerce raiding—strategy. Additionally, the Anglo-American victory in May 1943 illustrated how the British and American navies employed new tactical concepts, backed by key technological advances, to finally secure the most crucial logistical route of World War II.
Pursued as a necessity due to Anglo-American naval superiority, a commerce-raiding strategy represented Germany’s only effective method to exercise sea control. This required Dönitz to target the transatlantic convoy system with merchant tonnage sunk as the primary metric of success. Characterized as “the supply train and reinforcement column of the sea,” transatlantic convoys were distinguished by their point of origin and their maximum sustainable speed. Eastbound convoys, labelled SC for slow and HX for fast, originated from North America while westbound convoys—ONS for slow and ON for fast—formed in British waters. Speed limited, ONS and SC convoys comprised vessels that could not maintain a speed of 10 knots or more. Samuel Elliot Morison, the official historian of the U.S. Navy in World War II, describes that a typical transatlantic convoy “consisted of 45 to 60 merchant ships steaming in nine to twelve columns, with 1000 yards between the columns and 600 yards between ships. A nine-column convoy would, therefore, present a frontage of four nautical miles and depth of one and a half miles or more, depending on the number of ships.” Escorted by fast destroyers, frigates, and corvettes, convoy escorts formed defensive screens on the periphery of the formation, and employed conventional depth charges, surface gunfire, or the recently-developed “Hedgehog”—a forward-firing mortar system that shot up to 24 bomblets—to attack U-boats. Furthermore, as the war progressed, very long range (VLR) bombers and patrol aircraft from the Royal Air Force, Royal Canadian Air Force, U.S. Navy, and U.S. Army Air Forces substantially increased convoy defense and hounded any U-boat unlucky enough to be caught on the surface.
Convoy ONS-5, a slow westbound convoy of 40 merchant vessels and two oilers, took station off the island of Islay, west of the Scottish coast, and headed into the North Atlantic on 22 April 1943. Led by Commodore J. Kenneth Brook, a Royal Navy veteran, ONS-5’s passage began on a northwesterly course toward 61˚45′N—a waypoint roughly halfway between Iceland and Greenland. Once there, ONS-5 altered its course to the west toward Cape Farewell, Greenland, followed by a final course change to the southwest for the run across the Grand Banks of Newfoundland and down the East Coast of North America. Escorted by British Escort Group B-7, under the command of Commander Peter Gretton, RN, ONS-5’s escorts consisted of eight vessels: the destroyer Duncan, the frigate Tay, the corvettes Sunflower, Snowflake, Loosestrife, and Pink; and two antisubmarine trawlers, Northern Gem and Northern Spray. 
Marc Milner, a professor of history at the University of New Brunswick, describes “three key problems associated with U-boats trying to attack a convoy in the broad ocean: initial location, assembly of the pack around the convoy, and the actual attack.” Dönitz’s solution to convoy detection was to assemble submarines into various groups, or “wolf packs,” on patrol lines perpendicular to the main transatlantic shipping routes. A complex undertaking, Dönitz exercised operational control over his deployed U-boats through Enigma-encrypted radio communications from Befehlshaber der Unterseeboote (BdU) inside German naval headquarters in Berlin. After initial contact, doctrine called for the first U-boat(s) in visual range to shadow the convoy and transmit an electronic beacon that drew in additional submarines. Regular convoy position, course, and speed updates filtered back to BdU, who then directed additional wolf packs toward a convoy intercept course. Finally, after massing as many submarines as possible, Dönitz’s wolf packs “were cut loose from their wireless ‘leashes’ and the ‘sea wolves’ earned their nickname.” Operating primarily at night, and almost exclusively on the surface, U-boats attacked in waves, moving through the escort screen at high speeds to fire as many torpedoes as possible before either escaping astern of the convoy or submerging beneath the convoy’s wake.
Buoyant over his successes against Convoys SC122 and HX229, Dönitz increased the operational tempo in the latter half of April 1943. Six days after the departure of ONS-5, 15 U-boats of patrol Group Star took station along a north-south line approximately 420 miles east from Cape Farewell, while 19 U-boats of Group Specht patrolled a northwest-southeast line nearly halfway between Cape Farewell and the Flemish Cap off the Grand Banks of Newfoundland. Additionally, Dönitz directed 13 U-boats of Group Amsel to take station on a line southeast of Specht, thereby bringing the total number of U-boats arrayed against ONS-5 along the northern trade routes to 47.
Battling the elements for days after its departure, ONS-5 rendezvoused with three additional merchantmen and their escort, the destroyer Vidette, on 26 April. Sighted by U-650 of Group Star in the early hours of 28 April, Duncan and Tay quickly received electronic indicators from one of the most effective Allied countermeasures developed during the war. An offshoot of the concept of navigational radio beacons, U-boats used the electromagnetic spectrum to either transmit encrypted messages back to BdU through high-frequency radios or to broadcast homing beacons for additional wolf packs. Conscious of the tremendous difficulty of deciphering Enigma-encrypted communications, Allied scientists developed a high-frequency direction-finder (HF/DF), or “Huff-Duff,” which enabled surface vessels and coastal stations to follow a line of bearing to the origination of a U-boat’s transmission. With a maximum range of 20 nautical miles, multiple HF/DF-capable escorts could triangulate a suspected U-boat’s position and attack on multiple axes.
Forewarned, Gretton and his squadron executed an innovative antisubmarine tactic developed by the Royal Navy, which leveraged HF/DF. Labeled “scare” tactics, naval escorts would run “down the bearing of HF/DF locations” at high speeds, forcing “one stalker after another to submerge,” thereby impeding a clear firing solution. Cued by multiple HF/DF indicators throughout the night, ONS-5’s escorts successfully fended off attacks from four U-boats of Group Star. Yet, on the morning of 29 April, in a rare submerged attack, U-258 drew first blood after evading the screen and torpedoing the American freighter McKeesport in the center of the convoy.
In addition to HF/DF, the British and American navies swiftly exploited the emergent capabilities of radar and sonar for convoy defense. Radar—the ability to send, receive, and process the returns of a radio wave transmitted in the atmosphere—permitted surface vessels and aircraft to turn the natural environment of the ocean into an advantage. Perfected by British scientists in 1940 with the development of the cavity magnetron, the Type 271M radar became the standard radar system deployed by British naval escorts. Operating with a wavelength of 10 centimeters, hence known as “10-cm radar,” the Type 271M enabled surface vessels to detect a surfaced or broached U-boat out to four nautical miles. Quick to follow, radar became a significant advantage for VLR bombers and patrol aircraft. Similar in design to the Type 271M, the ASV Mark III radar “enabled aircrews to locate U-boats operating below the thick cloud cover . . . and ambush them by suddenly dropping through the cloud layer to straddle the U-boat with a pattern of aerial depth charges.” Limited compared to modern systems, radar drastically increased the capability of sonar-equipped naval escorts—a scientific evolution initially deployed by the U.S. Navy in 1916 and further advanced by the Royal Navy during the interwar period—which allowed an operator to track a submerged object through the propagation of sound in the water.
Alerted to the presence of several wolf packs near ONS-5’s position, Western Approaches Command, under the command of Admiral Sir Max Kennedy Horton, rushed to implement a ground-breaking tactical concept in antisubmarine warfare. Horton believed that a lack of sufficient resources, both surface vessels and aircraft, allowed Dönitz to mass large U-boat concentrations against weakly defended convoys. Promoted to his current position in the fall of 1942, Horton diverted a number of his escort vessels from convoy duty and deployed them as offensive support groups; the forerunners of modern surface action groups. Based out of Newfoundland and Iceland, and organized into five squadrons of destroyers, frigates, and corvettes, Horton expanded the ability of the Allies to quickly reinforce convoy escorts in extremis. Reinforced by the destroyer Oribi on 29 April, Horton’s 3rd Support Group—comprised of the destroyers Offa, Impulsive, Panther, and Penn—sortied from Newfoundland that day, and rushed toward ONS-5’s position on the periphery of the Greenland air gap.
The revolutionary potential of airpower in naval warfare became apparent to both the Allies and the Axis powers shortly after the war began, yet until the spring of 1943, Allied aircraft range limitations created a deadly gap in coverage in the mid-Atlantic. Dubbed the “Greenland air gap,” U-boats quickly took advantage of this crucial weakness in the transatlantic convoy system. Concentrated in force in the mid-Atlantic, U-boats racked up a stream of incredible successes, illustrated by Convoys SC122 and HX229, yet Allied industrial and scientific minds rose to the challenge in modifying the Liberator III—as the American Consolidated B-24 Liberator bomber was known in British service. Deployed to air bases in North America, Greenland, Iceland, and the British Isles, engineering modifications enabled the Liberator III to expand air coverage over transatlantic convoys. With an increased combat radius of 2,300 nautical miles, the Liberator III remained the only Allied aircraft capable of patrolling the Greenland air gap until the advent of escort carriers in late May 1943.
A harsh North Atlantic gale gave way to a heavy fog on 2 May, and Gretton became increasingly concerned over the fuel state of his escorts. The 3rd Support Group, under the command of Captain J. A. McCoy, RN, rendezvoused with ONS-5 that afternoon, yet low fuel forced Impulsive to break away and head toward Iceland. Further, fuel considerations compelled Duncan, Gretton’s flagship, to detach from escort duty on 3 May, and proceed toward Newfoundland. Lieutenant Commander R. E. Sherwood, RN, commander of the frigate Tay, assumed control of ONS-5’s escorts, but the weather continued to worsen on 4 May, and low fuel levels again drove an additional two destroyers from the 3rd Support Group to detach and return home. Weakened at a crucial hour, Western Approaches Command ordered the 1st Escort Group—comprised of the sloops Pelican and Sennen, and the frigates Jed, Wear, and Sprey—to sortie from St. John’s and make best speed toward the soon-to-be surrounded convoy.
The 48-hour period from 4–6 May marked the decisive moment for ONS-5, the Allied convoy system, and Germany’s guerre de course strategy. On 4 May, Dönitz dissolved the remnants of Group Star and reoriented them between Cape Farewell and the Flemish Cap. Thirty U-boats, formed into a new wolf pack “Fink,” straddled a line at the 55th parallel so as to block any southward movement of ONS-5, while BdU subdivided 21 U-boats of wolf pack Amsel into four groups astride a north-south line east of the Flemish Cap along the 51st parallel. The weather, previously a thorn in the side of ONS-5, subsided on the afternoon of 4 May, and allowed the Royal Canadian Air Force to join the fray. Sighting first U-630 and then U-438, VLR aircraft pounced on both marauders and forced them to break off the engagement. As night approached, the battle for ONS-5 began in earnest approximately 300 nautical miles southwest of Cape Farewell.
Group Fink, in small groups of several U-boats, launched a series of attacks on vulnerable stragglers astern of the convoy’s formation. ONS-5’s escorts took advantage of radar, sonar, and HF/DF cues to run down one U-boat after another, but by dawn on 5 May, seven additional merchant vessels were lost. Still, the destroyer Oribi continued to aggressively harry several U-boats astern of ONS-5 and successfully sank U-192 with its Hedgehog mortar, but the American tanker West Maximus succumbed to torpedoes fired from U-548. Shifting toward the convoy’s main body, ONS-5 endured multiple attacks. Although four additional merchant vessels were lost, the destroyer Offa severely damaged U-266, thereby forcing the unlucky U-boat to break off the engagement. With the capability for one more attempt as darkness approached on 5 May, Dönitz ordered his U-boats to continue to press the engagement regardless of air coverage—a break in U-boat doctrine—and reoriented 15 U-boats of Group Fink ahead of ONS-5’s track.
Shielded by a fortuitous fog, ONS-5’s escorts shattered Dönitz’s final attack. With gunfire, the corvette Loosestrife forced U-267 to submerge early in the evening and later sank U-638 with depth charges after avoiding a spread of torpedoes. Ordered to assist the corvettes, Oribi sighted a U-boat moving out of the fog off its starboard bow and rammed U-125, which was later sunk by gunfire from the corvette Snowflake. Refusing to ease the pressure in the early hours of 6 May, the destroyer Vidette sank U-531 with a Hedgehog attack, while Sunflower rammed and drove off U-533. Reinforced by the 1st Escort Group shortly before dawn, the sloop Pelican dispatched U-438 with gunfire while the sloop Sennen forced U-267 to withdraw from contact. When the fog lifted that morning, Dönitz called off his hunters, unwilling to face imminent air coverage from VLR aircraft out of Newfoundland. A day later, ONS-5 reached the predetermined Western Ocean Meeting Point, and Sherwood turned over escort duties to local forces for the remainder of the convoy’s voyage.
In the course of its 16-day passage, ONS-5 confronted the largest concentration of U-boats assembled against a single convoy during the war. Out of an initial complement of 40 merchant vessels and two tankers, the Germans sank 13 ships for a loss of seven U-boats, an unsustainable ratio for Germany. In his order withdrawing all U-boats from the North Atlantic on 24 May, Dönitz explained that “losses, even heavy losses, must be borne when they are accompanied by corresponding sinkings. But in May in the Atlantic, the sinking of about 10,000 GRT had to be paid for by the loss of a boat, while not long ago there was loss only with the sinking of about 100,000 GRT.” The ratio of 10,000 tons sunk per U-boat lost illustrated a remarkable turnaround in Allied fortunes. Dönitz, in his memoirs, concluded that “radar, and particularly radar location by aircraft, had to all practical purposes robbed the U-boats of their power to fight on the surface.” Correct to an extent, Dönitz’s assessment of the U-boat’s defeat in May 1943 is incomplete, and warrants further analysis.
In the duel between convoys and U-boats, British and American scientists played a crucial role in the development and deployment of key technological advances that gave the Allies a considerable edge in combat. The deployment of the Type 271M and ASV Mark III radars enabled Allied warships and aircraft to contest the natural stealth advantage of a surfaced U-boat, while HF/DF exploited a significant weakness in Dönitz’s wolf-pack tactics. Unlike U.S. Navy submarines in the Pacific, which operated independently from headquarters once on patrol, U-boat doctrine required daily contact with BdU, and mandated that, once in contact with a convoy, U-boats transmit electronic beacons which became key indicators for any escort commander as demonstrated during the engagements of 4–6 May. In a modern context, the U.S. Navy’s substantial reliance on network-centric warfare creates critical vulnerabilities, which a moderately capable adversary could leverage to its advantage. If skilled in the manipulation of the electromagnetic spectrum or cyber domain, a nation could theoretically not only constrain a U.S. carrier strike group’s freedom of action but could also target unprotected merchant vessels, and bring sufficient pressure to bear on key U.S. sea lines of communication.
In a commerce-raiding strategy, Dönitz correctly deduced that the key success metric was tonnage, yet he was unable to apply enough force at crucial moments between March and May 1943. Captain S. W. Roskill, RN, the Royal Navy’s official historian, indicated that “as long as the enemy was sinking more ships than we were building, the final victory would remain in the balance—as the Germans very well realized.” The industrial disparity between the Allies and Axis powers, over time, severely limited Dönitz’s offensive on the Atlantic trade routes, a factor that became evident in mid-1942 when new construction surpassed tonnage lost to U-boat activity. As one historian writes, “Germany’s only chance to bring Britain to her knees . . . had been in the early years of the war . . . before America’s prodigious industrial prowess had come to play.” Even if confronted by America’s massive shipbuilding effort, German submarines still managed to rack up incredible successes in the Atlantic as evidenced by 7,038,166 tons of shipping sunk between January 1942 and April 1943. Still, in a fundamental strategic error, the allocation of German industrial resources toward submarine construction did not reach full-scale production until well after the start of the war. Although construction peaked in 1944, the increasing demands of the Eastern Front severely constrained German industry.
Ultimately, the battle over Convoy ONS-5 demonstrated a fundamental premise of sea power. The influential naval strategist Julian Corbett theorized that since “you cannot conquer [the] sea because it is not susceptible of ownership,” maritime superiority must mean “nothing but the control of maritime communications.” If, as Overy suggested, sea power remains one of the only ways the United States can effectively wage a global war, then the principal objective of a navy must be to secure key sea lines of communication for follow-on operations. Dönitz’s pursuit of a guerre de course strategy against Great Britain ultimately failed, but the theoretical foundations of this strategy remain sound—as demonstrated most effectively by U.S. submarine operations in the Pacific theater during World War II. Undoubtedly, the Allied victory in the North Atlantic paved the way for the eventual defeat of Nazi Germany. As Morison keenly observed, “without victory over the U-boat, the American men and material who contributed to the defeat of Germany could never have reached the theater of conflict, and in all probability the British Isles would have been blockaded and starved into submission.” Regardless of the current budgetary constraints faced by the U.S. Navy, the oceans of the world remain the primary mode of transport for the global economy—according to the International Maritime Organization, more than 90 percent of global trade will travel by sea in 2017. As the Battle of the Atlantic illustrates, the ability to either control or deny access to these vital maritime trade routes guarantees strategic flexibility and can very well mean the difference between victory or defeat in future conflicts.
Chalmers, S. William. Max Horton and the Western Approaches: A Biography of Admiral Sir Max Kennedy Horton. 3rd Ed. London: Hodder and Stoughton, 1954.
Corbett, S. Julian. Some Principles of Maritime Strategy. 1918. Reprint, United Kingdom: Naval and Military Press, 2009.
Dönitz, Karl. Ten Years and Twenty Days. Translated by R. H. Stevens. London: Weidenfeld and Nicolson, 1959.
German Naval Staff. Befehlsbaber der Unterseeboote (BdU) War Diary, Volume 7: 1 Jan 1943–30 Jun 1943. Translated into English, Washington DC: Naval Historical Center, Special Collection Oversize.
Miller, Nathan. War at Sea: A Naval History of World War II. New York: Oxford University Press, 1995.
Milner, Marc. “The Battle That Had to Be Won” Naval History 22, no. 3. (June 2008).
Morison, Samuel E.. History of United States Naval Operations in World War II, Volume 1, The Battle of the Atlantic: September 1939–May 1943, 2nd ed. Annapolis, MD: Naval Institute Press, 2010.
———. History of United States Naval Operations in World War II, Volume 10, The Atlantic Battle Won: May 1943–May 1945, 2nd ed. Annapolis, MD: Naval Institute Press, 2010.
Offley, Ed. Turning The Tide: How A Small Band of Allied Sailors Defeated the U-boats and Won The Battle of the Atlantic. New York: Basic Books, 2011.
Overy, Richard. Why The Allies Won. New York: Norton & Company Inc., 1997.
Roskill, S. W. Captain, RN, The War at Sea: 1939–1945, Volume 2, The Period of Balance. London: Her Majesty’s Stationary Office, 1956.
Syrett, David. The Defeat of the German U-boats: The Battle of the Atlantic. Columbia: University of South Carolina Press, 1994.
 David Syrett, The Defeat of the German U-boats: The Battle of the Atlantic (Columbia: University of South Carolina Press, 1994), 25.
 Quoted in Nathan Miller, War at Sea: A Naval History of World War II (Oxford: Oxford University Press, 1995), 343.
 Richard Overy, Why the Allies Won (New York: Norton & Company Inc., 1997), 18.
 Samuel Elliot Morison, History of United States Naval Operations in World War II, Volume 1, The Battle of the Atlantic: September 1939–May 1943, 2nd ed. (Annapolis, MD: Naval Institute Press, 2010), 17.
 Morison, Volume 1, 19.
 Ed Offley, Turning The Tide: How a Small Band of Allied Sailors Defeated the U-boats and Won the Battle of the Atlantic (New York: Basic Books, 2011), xii–xiv.
 Syrett, 65.
 Ibid., 64.
 Marc Milner, “The Battle That Had to Be Won,” Naval History 22, no. 3, June 2008.
 Samuel Elliot Morison, History of United States Naval Operations in World War II, Volume 10, The Atlantic Battle Won: May 1943–May 1945, 2nd ed. (Annapolis, MD: Naval Institute Press, 2010), 67.
 Offley, 72.
 Morison, Volume 10, 68.
 Offley, 59–60.
 Morison, Volume 1, 213.
 Offley, 274.
 Ibid., 46–47.
 Morison, Volume 10, 75; Captain S. W. Roskill, RN, The War at Sea: 1939–1945, Volume 2, The Period of Balance (London: Her Majesty’s Stationery Office, 1956), 373–4.
 Morison, Volume 10, 70–71.
 Ibid., 74.
 Ibid., 75.
 German Naval Staff, Befehlsbaber der Unterseeboote (BdU) War Diary, Volume 7: 1 Jan 1943 – 30 Jun 1943, trans (Washington D.C.: Naval Historical Center), 429.
 Admiral Dönitz, Ten Years and Twenty Days, trans. R. H. Stevens (London: Weidenfeld and Nicolson, 1959), 341.
 Roskill, 379.
 Miller, 348.
 Merchant and Neutral shipping loss statistics derived from Morison, Volume 1, 410–14.
 Julian S. Corbett, Some Principles of Maritime Strategy (1918; repr., United Kingdom: Naval and Military Press, 2009), 79–80.
 Morison, Volume 10, 361.