Wolf Packs: Battle of the Atlantic
History of Naval Warfare, Part 15
One of the hallmarks of the Second World War was the technological maturity and systematic application of military technologies which were still in their infancy during the first war. Tanks, which had previously been lumbering and mechanically histrionic death traps, emerged as pivotal assault and exploitation weapons which trampled Europe by the thousands. Aircraft, initially used in the first war in reconnaissance roles, now swarmed in vast hordes, ranging hundreds of miles into enemy space and disgorging unprecedented firepower. The radio became ubiquitous on the battlefield and provided unprecedented levels of command and control over far flung and fast moving units.
Tanks, mechanized infantry forces, self propelled artillery, rockets, strategic bombers, close air support: all deadly and cinematic, and part of a deadly new tactical package. However, they could hardly match the sinister terror induced by the most understated and subtle member of this maturing era of warfare: the submarine.
The Second World War witnessed two concurrent campaigns by which submarines were used in an attempt to economically isolate and degrade an island nation enemy. One of these attempts was remarkably successful. In the Pacific, US Submariners sunk millions of tons of Japanese shipping - more shipping, in fact, than Japan had possessed at the outbreak of war. A brutally effective submarine campaign against Japanese tankers affected a near perfect starvation of Japan’s war machine: after intaking 40% of East Indies crude production in 1942, only 5% would reach Japanese shores in 1944. This was a cataclysmic decline which Japan could not survive, owed largely to the 155 tankers sunk by American submarines in 1943 and 1944. In the final year of the war, American boats were able to undertake the ultimate dream of submarine theorists: a close blockade of the Japanese home islands, with American submariners prowling practically every inlet and bay.
The success of the American submarine campaign was genuinely astonishing, and created a near perfect asphyxiation of the Japanese war economy, with imports of virtually every vital industrial input plummeting to near zero by 1944. Admiral Charles Lockwood, who commanded the Submarine Force Pacific Fleet, was probably only slightly boasting when he later told an instructor at the Naval Academy:
Now don’t teach those midshipmen that the submariners won the war. We know there were other forces fighting there, too. But if they kept the surface forces and the flyboys out of our patrol areas we would have won the war six months earlier.
Despite the phenomenal success of America’s submarine operations against Japan, the American war on Japanese shipping generally receives scant attention. To take just one example, Francis Pike’s magisterial and colossal tome on the Pacific War relegates American submarine operations to an appendix. In contrast, there is an astonishing volume of literature devoted to the war’s other grand submarine campaign: the so-called Battle of the Atlantic. Germany’s famous U-boats attempted a similarly strategic interdiction war against shipping to the British home isles. Unlike the American submarine force in the Pacific, however, the U-boats failed.
Germany’s U-boat campaign against Britain has always been lavishly furnished with attention, not only because of its intrinsically interesting qualities - with at times hundreds of submarines at sea hunting across a battlespace of more than 10,000 square miles - but also because it seemed to offer one of Germany’s few genuine levers against Britain, and therefore one of the few true avenues through which Germany may have won the war. Winston Churchill famously noted in his memoirs that “The only thing that ever really frightened me during the war was the U-boat peril”, implying that the war on shipping really had some element of war-breaking potential.
The U-boat campaign was, to be a sure, an important element of the emerging global war. As a major departure point for alternative histories, however, the Battle of the Atlantic is always bound to attain some element of controversy. It is increasingly popular, in the historiography, to focus on the sheer magnitude of American industrial power and conclude that there was simply nothing that the German U-boat force could really do to plug the flow of shipping across the Atlantic. In this view, the Germans were fighting a brutal but ultimately futile delaying action against an unstoppable economic force, and had no better prospects of success than a man trying to plug a failing dam with his fingers. The general presumption is that Germany never had real prospects of winning the global war, which renders alternative history a waste of mental energy. It bears repeating, however, that the men running the war against Germany - men like Churchill and the British Admiralty - viewed the U-boats in real time as a genuinely lethal threat. Dismissing this as a folly exposed by brute industrial statistics would be a mistake. Both German and British naval leadership saw the Battle of the Atlantic as a genuinely decisive aspect of the war, and we should do them the courtesy of trying to see what they saw.
Hitler’s British Problem
One of the most striking and nearly ubiquitous problems with the popular historiography of the Second World War is the practice of dramatically exaggerating Great Britain’s strategic peril in the aftermath of French defeat in 1940. From films like Dunkirk and Darkest Hour, to trite popular histories like The Splendid and the Vile, the common practice is to portray a tottering and beleaguered Britain, alone and under siege by merciless Luftwaffe bombing, staring down the barrel at cataclysmic defeat. The Churchill cult always works to play up this perception, emphasizing a creeping fog of defeatism and crisis which was transcended only by the irascible courage of the hard drinking and hard driving prime minister.
Rarely is Britain’s strategic position considered from the German point of view. As the Germans saw it, Britain was not a battered and besieged state in crisis, but rather a great porcupine loitering offshore which they had no clear way to strike at. Britain retained a significant and growing air force, an enormous industrial plant, economic linkages to a vast overseas resource base (both in its own empire and in the United States), and absolute supremacy at sea. So long as Britain chose to keep fighting, Germany had shockingly few direct kinetic levers against her.
The Luftwaffe’s strategic air campaign - the famous Battle of Britain - is an ideal example. The usual impression given is that the German air force was really on the verge of bringing the RAF to the breaking point in 1940, but this was largely a mirage stemming from poor intelligence on both sides. German intelligence tended to drastically underestimate British aircraft production, giving the impression that the Royal Air Force was close to defeat when it was not. For example, in 1940 Luftwaffe command staff estimated British aircraft production at 9,900, of which 2,790 were thought to be fighters. Actual British output that year clocked 15,049 aircraft, of which 4,283 were fighters. German intelligence also drastically overestimated the efficacy of their bombing and believed that British production would decline to some 7,000 aircraft in 1941; in fact, British production was accelerating and would top 20,000 planes that year.
Without being drawn too far into the particulars of German memorandum, the basic fact is that throughout the latter half of 1940 the Luftwaffe generally believed that the RAF was taking delivery of far fewer aircraft than it really was, and incorrectly concluded that it was on the verge of air supremacy over southern Britain. This caused a sense of disillusionment and mild bewilderment when, contrary to expectations, RAF Fighter Command continued to throw aircraft into combat rather than collapsing. The high hopes placed on the strategic air campaign faded gradually and plans for an amphibious landing in Britain were quietly shelved.
On the other hand, British intelligence had the opposite tendency to overestimate German strength. This was partially because the ratio of operational craft and reserves in the Luftwaffe was incorrectly presumed to be in-line with British practice (leading to the belief that the Germans had far more reserve aircraft than they really did), and partially because British intelligence greatly overestimated German production. An August report from British Air Intelligence estimated German output of 24,400 aircraft in 1940, with a front line strength of some 5,800. In fact, German aircraft output that year was just 10,247, with a serviceable frontline strength of just 2,054.
These numbers reduce to a fairly simple problem. The Germans underestimated British aircraft production by 50%, and the British overestimated German production by 140%. Consequentially, the British believed they were fighting a desperate struggle against an overwhelming enemy, and the Germans believed they were working to finish off an overmatched foe that was all but defeated. Add it all up, and everybody apparently agreed that the RAF was in serious trouble. But this was never really the case, and the “Battle of Britain” became a sort of mutually attritional struggle that was never particularly decisive. By October 1940, when it was finally becoming clear that the Luftwaffe had failed to gain air supremacy, both side had roughly 700 operational fighter aircraft with adequate pools of trained pilots. Neither side was really defeated, but the stalemate in the air left Germany without a mechanism to strike directly at Britain on any meaningful scale.
German strategic dissipation proceeded rapidly from this point. Planning for Operation Barbarossa began to dominate resource allocation problems in 1941, and the breakdown of the campaign in the east had further implications for the emerging war against the Anglo-American bloc, both in terms of delaying the consolidation of Germany’s blockade-proof continental economic space, and changing the nature of Germany’s strategic perspective in the west. By 1942, the focus had shifted from finding a mechanism to defeat Britain outright towards a method for preventing the opening of a second front in the west.
Enter Karl Dönitz. The navy’s Befehlshaber der Unterseeboote - “Commander of the U-boats” - Dönitz became the originator and advocate of a particular theory of the war against allied merchant shipping. Dönitz identified allied seaborne transport capacity as the critical strategic problem facing the Anglo-Americans and therefore the critical objective for the navy to degrade.
The Kriegsmarine had been operating against allied shipping from the beginning of the war using both surface warships and U-boats; in fact, a memorandum from Hitler the day before the invasion of Poland instructed the Navy to “operate against merchant shipping, with England as the focal point.” Nothing about Dönitz’s ideas were particularly novel or interesting from that perspective. What was new, however, was Dönitz’s twofold assertion, first the objective of the navy’s operations was to very explicitly to sink the highest aggregate tonnage of enemy shipping, rated against expected construction, and secondly that this could only be achieved by U-boats.
Dönitz, very particularly and personally, was the originator of the mathematic framework for the U-boat war: the idea that the Anglo-Americans could be attritted and perhaps even defeated provided that their shipping tonnage was sunk at a rate which exceeded new construction. The implication of this was that the navy needed to adopt tactics that aimed to sink the absolute maximum tonnage of shipping. In practical terms, what this meant was that U-boats ought not to be positioned based on other operational considerations (like the defense of the Norwegian coast, or interdiction in the Mediterranean) - rather, submarines needed to be in the places where they could sink the most shipping tonnage. In April, 1942, Dönitz wrote:
The enemy merchant navies are a collective factor. It is therefore immaterial where any one ship is sunk, for it must ultimately be replaced by new construction. What counts in the long run is the preponderance of sinkings over new construction. Shipbuilding and arms production are centered in the United States, while England is the European outpost and sally port.
The tonnage war had three sublime factors recommending it. The first was the idea that its success could be guaranteed by attaining quantified and measurable objectives: if the Germans could succeed over an extended period of time in sinking more ships (tonnage equivalent) than the Anglo-Americans could build, then the British war economy would inevitably degrade until it was crippled and eventually collapsed. Secondly, the tonnage war had both an offensive and defensive facet to it: not only did it offer a path to collapse the British war economy, but it would also degrade the Anglo-American capacity to convey war material from America to Britain. This would delay the accumulation of American ground forces in Europe and by extension delay the opening of a second front in France or Norway. Finally, the tonnage war could be waged, according to Dönitz, exclusively by U-boats, which unlike surface warships could be brought online relatively quickly and based safely on the French Atlantic coast.
This latter point was particularly important. After the sinking of the Bismarck, no German capital ships ever ventured back into the Atlantic, and by the time Dönitz began his aggressive push for the tonnage war, the Germans had already implemented Operation Cerberus to recall the remaining battleships from the French Coast. This was because the Atlantic bases had proved remarkably vulnerable to British air strikes. Submarines, however, being significantly more compact, could maintain their basing in France under the shelter of fortified, bomb proof pens which could not be built for larger surface warships. Indeed, the RAF would repeatedly bomb the U-boat bases on the French coast and come away mildly astonished at how durable they were.
Even more importantly, U-boats - unlike surface ships - could attack even well escorted convoys. According to Dönitz, surface vessels like cruisers could not freely attack shipping lanes because they had a fundamentally “defensive” priority of evading superior enemy forces. As a memo from Dönitz’s staff put it:
Only the U-boat can therefore continue to penetrate into the areas where the enemy enjoys naval supremacy, remain there, and fight, since it does not need to take issue with this enemy supremacy. The increased presence of enemy battleships and cruisers in these areas does not mean greater danger for the U-boat, but on the contrary a welcome increase in targets. The Commander of the U-boats emphatically disputes that our battleships and cruisers are indispensable for the conduct of the war in the Atlantic.
All of this leads, in a roundabout sort of way, to a fairly basic question, as to what exactly the “Battle of the Atlantic” was, and when it took place. “Officially” - noting fully the sarcasm implied by the quotation marks - the Battle of the Atlantic lasted the entire duration of the war, with German submarines at sea and engaged in combat operations up until the literal day of German surrender on May 8, 1945. Similarly, U-boats were underway and conducting attacks on allied shipping in 1939 and 1940, and the Germans began establishing bases on the French Atlantic coast within weeks of French surrender. If the “Battle of the Atlantic” is taken to mean the entire sweep of German U-boat operations in the Atlantic, then in fact it did cover the duration of the entire European war and was among the longest and most complex naval operations in history.
A more meaningful dating schema places the critical action in the Atlantic in a two year period from May 1941 to May 1943. On May 8, 1941, Dönitz made the fateful decision to widen the area of U-boat operations. Formerly, U-boat operations had been limited to patrol lines on the lanes of approach into the British Isles, but the order of May 8 laid the ground work for attacks on convoys in the open North Atlantic. The area of operations would eventually widen farther to include America’s eastern seaboard, after the formal entry of the United States into the war. Some two years later, on May 24, 1943, Dönitz called a halt to such attacks, on the grounds that the loss of submarines had risen to an “intolerable level.” A more precise estimation of events would thus date the Battle of the Atlantic from May 8, 1941 to May 24, 1943. Not coincidentally, this also coincided with the period in which the U-boat force in the Atlantic consistently grew. In May 1941, there were just 24 boats operating in the Atlantic on average. This number grew steadily through the end of the year before exploding in 1942 as an accelerated building program kicked in, before peaking in May 1943 with an average at-sea force of 118 boats - a total which thereafter declined.
Operational trends varied greatly across those crucial years of the submarine war. Of course, the material basis of the U-boat campaign shifted substantially as U-boat designs and allied countermeasures improved. The situation escalated with particular rapidity in 1942, both because of American entry into the war and because it was not until this year that German submarine production really began to reach impactful levels. Furthermore, the locus and intensity of the U-boat operations would ebb and flow based on both opportunity and strategic priorities. In February and March of 1942, for example, the area of operations shifted into the Caribbean, with German submariners drawn by both the relatively balmy weather and the crude tankers sailing from Venezuela. Ancillary European theaters like the Mediterranean and the Black Sea also absorbed resources, and Dönitz was always forced to keep more U-boats around Norway than he would have liked, simply to satisfy Hitler - who spent years preoccupied with a supposed British reinvasion of Scandinavia which never came.
Regardless of all the particulars and the distractions, for Dönitz U-boat operations took on the character of an absolute war against allied tonnage. Because shipping tonnage was viewed as an essentially fungible, or interchangeable, capability, for Dönitz the matter was simple: U-boats needed to operate at scale in the areas where they could sink the most tonnage in absolute terms, including the heavily trafficked routes on the North Atlantic. In the spring of 1942, he estimated that the United States and Great Britain could collectively build 8.1 million GRT (Gross Register Tonnage) in 1942, increasing to 10.3 million GRT in 1943. On this basis, Dönitz argued: “We would have to sink approximately 700,000 tonnes per month in order to offset new constructions.” Provided the U-boats could consistently achieve this mark over a sustained period of time, the enemy war economy would *necessarily* degrade and eventually collapse.
The result was a peculiar sort of war predicated on accounting statements. Dönitz and his staff assessed the results of the U-boat war as a function of two simple ratios: tonnage sunk relative to expected allied construction, and U-boats lost relative to completions. Since an accelerated submarine construction program was expected to yield some twenty new boats per month, and on the basis of his own estimates of allied shipbuilding, the math of this war of industrial attrition was simple: if the U-boat force could sink more than 700,000 tons of shipping per month while keeping losses to a minimum, then allied shipping would inexorably degrade while the submarine force grew in strength. If this happened, Germany would be winning.
Pack Hunters
Karl Dönitz was an interesting character. Invariably described as a man of imposing intelligence, he was undoubtedly among the smartest and most organized men among the higher German leadership. One of the great peculiarities of his life was his inglorious (and mercifully brief) tenure as Hitler’s successor. When Nazi leadership collapsed at the end of April, 1945 - with Hitler and Goebbels committing suicide and Himmler and Goering branded as traitors - Dönitz was left holding the bag, posthumously named Head of State by the late Fuhrer. The Admiral’s postwar memoirs, titled Ten Years and Twenty Days, were a nod to his two high posts in service to the Reich: ten years as the commander of the U-boats, and twenty days as the president of a defeated Germany.
In any case, at Nuremburg he was found to be intelligent, lively, and relatively affable by Anglo-American personnel. He genuinely seemed to believe that the Anglo-Americans would be grateful to him for surrendering the U-boat fleet to them, rather than to the Soviets. This synergized with the experience of British personnel who took custody of surrendering U-boats after the war: they documented that many of the German crews were downright friendly, and asked the British when they would fight “the Russians” together.
If Dönitz was expecting gratitude, he was to be disappointed. His interrogation file notes that he was outraged at the suggestion that he could be tried as a war criminal, and maintained with total conviction that the navy had fought a clean war. Many allied officers agreed with him. US Admiral Daniel Gallery felt that Dönitz’s actions were consistent with American submarine warfare, and that his trial at Nuremburg was an “outstanding example of barefaced hypocrisy”, which was “an insult to our own submariners.” He later wrote that if he ever met Dönitz, “I would have trouble looking him in the eye. The only crime he committed was the crime of almost beating us in a bloody but legal fight.”
Dönitz was not pleased to be charged with war crimes, but on almost all other subjects, however, he proved happy to converse in an essentially friendly tone, and discussed topics like radar and experimental U-boat designs. The British Division of Naval Intelligence concluded:
Doenitz, classified by psychological tests as just below the genius class, is an independent thinker, clear and precise, and is an expert in his field.
The file also notes, with a tone of the ominous:
Doenitz is credited with having invented the “wolf pack” technique of submarine warfare.
Any discussion of Second World War U-boats inevitably runs straight into this intimidating and thorny term, which is generally viewed as Dönitz’s seminal tactical innovation. The general impression of the wolf packs is usually that of a tactical system which allowed allied convoys to be attacked by massed submarines, sometimes numbering a dozen or more. But this is not quite correct. The wolf pack was not a tactical system in the strictest sense, in that it did not allow for orderly command and control or synergistic movements during an attack. The wolf pack system was not really a question of tactics at all, but was instead closely related to significant German advances in signals intelligence and communications.
To understand what this means, we must return to the First World War and recall why convoys were such an effective answer to submarines in that conflict. Although escort ships in the first war had some successes deterring or sinking submarines, U-boats that encountered convoys were generally able to attack. The primary advantages of the convoy, rather, were primarily concealment and survivability. By concentrating ships into convoys, the Anglo-Americans were able to denude the sea of targets and make it much more difficult for U-boats to spot their prey. Additionally, although U-boats could usually attack convoys successfully and then escape, they generally had time to torpedo only one or two targets before hightailing it away. The majority of the convoy was expected to be unharmed, and more importantly they were on station to rescue survivors.
Doenitz’s wolf pack system was an element of a comprehensive answer to the convoy system which completely turned the logic of the convoy upside down. In comparison to the First World War, Doenitz’s U-boat force had two critical capabilities that had previously been sorely lacking: they could reliably locate convoys, and they could attack them at scale once they had been spotted. These advantages, however, derived primarily from improvements to communications and signals intelligence, rather than a tactical methodology as such.
The first step in overcoming the convoy system was devising a reliable method for locating convoys in the first place. In the First World War, the British had quickly discovered that a convoy containing dozens of vessels was not particularly easier to spot than a single vessel, and U-boats - with their low profiles and short conning towers - were bad at sighting far off targets. The benefits of aerial reconnaissance were obvious, but Germany’s sole suitable long-range reconnaissance craft - the Fw 200 Condor - was never available in suitable numbers. Efforts by the naval command to reinforce air reconnaissance were frustrated by both a shortage of aircraft and the truculent Goering, who was disinterested in cooperating with the navy.
Although Condor flights did contribute valuable reconnaissance on occasion, wide-ranging aerial surveillance was never systematically available to the Germans and could do little to offset the poor visual range of the U-boats at scale. However, the Germans did benefit tremendously from the great strides that they had made in signals intelligence, cryptography, and radio communications. The allies were famously successful at breaking German ciphers and reading the famous Enigma traffic. Far less famous was the parallel effort of the German B-Dienst office (short for Beobachtungsdienst, or observation service). This was a signals intelligence department in the German Naval Intelligence Service, which by the autumn of 1941 had broken the British Naval Combined Cipher, which provided a steady stream of clues concerning convoy sizes, positions, courses, and escorts.
The intelligence provided by B-Dienst allowed the German U-boat force to place patrol lines along the expected course of convoys at sea. The optimal practice was to position a large number of U-boats in a patrol line (with gaps of some 40 nautical miles between them) in the suspected path of the convoy. It was at this point that Germany’s system of signals traffic and wireless communication became crucially important. The first U-boat in the patrol line would not attack immediately, but instead fall in behind the convoy in a hidden trailing position, maintaining visual contact and calling in the remaining boats in the patrol line.
This was much more difficult than it sounded. The idea of summoning the entire patrol line to attack a convoy simultaneously sounds fairly obvious, and raises a question: why was Dönitz so highly regarded for devising such an elementary tactic? The answer, as such, is that while the tactic of a group or pack attack was generally quite obvious, it required a remarkable system of communication and control to actually implement it.
Operational control of the U-boats required extensive radio communications routed through Dönitz’s headquarters in France. Orders had to be given to form and route patrol lines, coordinate massed attacks on convoys, and then re-form attack groups. By 1943, when the Germans had in excess of 100 U-boats underway at any given time, Dönitz’s headquarters was handling well over 2,000 radio signals per day, all of which had to be encrypted and then repeated episodically to ensure that every boat received the relevant orders. In addition, communications officers on every U-boat had to receive and transcribe every single signal before decrypting it to find out if it was relevant to them. Radio traffic control is decidedly unsexy in the context of a global war, but Dönitz wielded a sophisticated and remarkably efficient signals network which was the key to making pack tactics possible. Sir Francis Harry Hinsley, a British intelligence officer who later wrote a magisterial multi-volume history of British intelligence during the war, made the assessment that the Kriegsmarine’s signals network was essentially unequalled in its complexity, efficiency, and flexibility.
All that is to say, the great achievement of the U-boat service was not that it discovered the advantages of attacking in groups (this had always been obvious), but rather a victory of organization and communication which made it possible for Dönitz, working out of his headquarters in the Villa Kerlilon in Lorient, to reliably direct dozens of U-boats to converge on spotted convoys thousands of miles away. Once a convoy was spotted by a U-boat in the patrol line, the boat would call in to headquarters, and the lithe and efficient German signals network would begin pulling other boats out of the patrol line to descend on the convoy.
The ideal, as such, was for all the boats within range to converge ahead of the convoy, massing in its path and loitering in anticipation of a night attack. The absolute ideal, although this could not always be executed, was for the U-boats to attack simultaneously at night from the “dark” side of the convoy, so that the enemy ships were silhouetted by the moon while the submarines were in darkness.
However - and here is a point of major confusion - there was no tactical control of the battle once the attack began. U-boats generally communicated very little once the action started, and neither Dönitz at HQ nor an officer on station exercised central control of the attack. Management of the battle was limited to taking a count of the U-boats on station, confirming that favorable conditions existed, and then commencing the attack. Once the attack actually began, each U-boat captain chose his own targets opportunistically and broke away as he saw fit, without external direction. The upshot of all this, and the sort of singular takeaway, is that U-boat pack hunting was not a tactical method for coordinating the attacks on convoys, but rather an operational-organizational system that allowed U-boats, spread out in wide patrol lines, to converge on their targets.
The irony of the U-boat war was that, although it increasingly came to be viewed as a decisive element of the broader war, it was a fight that neither the British nor the Germans were well prepared for. The British, on the basis of their success overcoming the U-boats in the First World War, did not consider submarines a serious threat to their sea transport. They considered convoying to be an essentially adequate solution and failed to anticipate the way that massed attacks could turn the logic of the convey upside down. Furthermore, the British were far too optimistic about the effect of new antisubmarine weapons like Asdic (early sonar) and depth charges. Asdic sonar soon revealed itself to be a terrifically flawed system. It had a limited range of a mile and a half at most, which left great gaps around the perimeter of convoys. Even more importantly, however, Asdic could not detect submarines on the surface, which rendered it meaningless in most attack scenarios. The biggest problem by far for the British, however, was a catastrophic shortage of escort ships. In the early years of the war, convoys of fifty or more ships traveling in 9 columns might have just 4 or 5 escorts, leaving huge gaps that were easy for U-boats to penetrate. Once submarine attacks commenced, escorts found it impossible to react correctly when outnumbered by the U-boat packs: wheeling to hunt for the offending submarine simply opened a new gap in the perimeter which was sure to be exploited by other boats.
That is not to say that the Germans were any better prepared than the British for an expansive U-boat war. The shortage of escorts gave the U-boats good tactical prospects when they fell on a convoy, but in 1941 there were simply far too few U-boats to capitalize on these opportunities at scale. The Germans were also hindered by their own technological blind spots, but whereas in the British case it was Asdic sonar that proved disappointing, the Germans were let down by their encryption.
The story of the German Enigma machines, the Ultra Project, Alan Turing, and the British cryptological project at Bletchley Park - although essentially unknown until the declassification of relevant materials in 1974 - are by now a fairly well known story. Thanks to a head start gifted by the Polish secret service (which had been studying German cipher machines since the 1920’s), their own herculean efforts, and the fortuitous recovery of German cipher materials, the basic fact is that the British were generally able to read U-boat wireless traffic in the Atlantic throughout 1941. The capture, intact, of the damaged boat U-110 complete with all its cipher materials, keys, and signal book was a particularly significant coup.
The most direct benefit from reading the U-boat traffic, from the British perspective, was not necessarily to hunt submarines (which still had tactical methods for escaping hunters), but to reroute convoys around U-boat patrol lines. This was achieved with significant success. Although the average number of U-boats in the Atlantic tripled between February and August 1941, the tonnage lost declined precipitously, so that July of that year saw the lowest losses since the fall of France. Dönitz was highly suspicious of the disappointing returns and suspected that the British were reading his mail, but an “investigation” by Naval Intelligence concluded that the Enigma system was fundamentally secure.
British signals intelligence did succeed in substantially curtailing shipping losses in the latter months of 1941. The German military historian Jürgen Rohwer estimated, based on the number of boats at sea, that the Germans reasonably expected to sink some 2,035,000 GRT in the second half of 1941, while actual sinkings, thanks to Ultra, were a mere 629,000 GRT: fully 70% below target. This was far too low to achieve a decisive result in the “tonnage war.” The basic ledger on 1941 was mixed, then. The British had learned that the convoy system, particularly given the paucity of escorts, was vulnerable to attack by massed U-boats, but they had blunted the worst of the damage by reading German radio traffic and evading patrol lines.
A variety of factors would converge to ensure that 1942 was the year in which the U-boat war began to accelerate and reach potentially decisive heights of intensity. Three important changes stand out above all. First and foremost, 1942 was the year where the at-sea submarine force actually began to grow to critical mass. Dönitz began 1941 with just 22 U-boats in the Atlantic on average, and by the end of the year this had climbed to 60 boats. In 1942, an accelerated construction program began to kick in, and the Atlantic force climbed to 160 boats (though not all of these would be at sea simultaniously). Secondly, in February 1942 the Germans added a fourth rotor to their naval cipher machines, which exponentially raised the complexity of the encryption and forced the British to work blind for the rest of the year. Finally, the entry of the United States into the war in the closing weeks of 1941 greatly expanded the operating areas of the U-boats in the following year.
American entry into the war opened up profitable new hunting grounds for the U-boats, largely courtesy of lax American defensive protocols. Traffic along the American seaboard was so vulnerable to U-boats, in fact, that Dönitz increasingly abandoned efforts against convoys in the open Atlantic to hunt along the American coastline, despite the longer trips required by. The reasons for weak American defenses were numerous. First and foremost, the US Navy had very few available escorts, and erroneously theorized that convoys without escorts were more vulnerable than ships sailing individually (deducing that an unescorted convoy created what amounted to little more than a shooting gallery for U-boats). The Americans also eschewed a variety of best practices suggested by the British, including coastal blackouts: instead, American cities remained brilliantly lit up, which nicely silhouetted ships at night for targeting.
More generally, it is fair to say that anti-submarine measures were simply not a top priority for Admiral Ernest King, who - to be fair - had plenty on his plate. A measure of arrogance vis a vis British advice, indifference by the overworked King, and a shortage of escort ships made the perfect mixture for defensive lethargy, and the result was a fantastic shooting spree for the U-boats along the American east coast, the Gulf of Mexico, and the Caribbean. In fact, from January to July 1942, the U-boats achieved their maximal rate of efficiency (taken as GRT sunk per at-sea submarine) and drastically reduced their losses by avoiding confrontations with escorted convoys. It was not until the summer of 1942, when the Americans belatedly introduced convoys along the east coast, that tonnage losses were stabilized, and Dönitz was forced to admit that a return to pack attacks against escorted convoys was now the only way forward.
The belated introduction of standard security measures and convoying along the American eastern seaboard led to an immediate drop in sinkings beginning in July, although U-boats would continue to hunt productively in the Caribbean where defenses were more lax. The collapse of the easy hunting along the American littoral precipitated a massive U-boat offensive against convoys in the North Atlantic, which began in August 1942. This is usually identified as the climax and final phase of the U-boat war.
