Military history

FIVE

War Supply and the Battle of the Atlantic

Supply of food, of raw materials, of finished products, of weapons themselves, lies at the root of war. From the earliest times man has gone to war to take possession of resources he lacks and, when at war, has fought to secure his means of livelihood and self-protection from his enemy. The Second World War was no exception to this rule. In the view of Professor Alan Milward, principal economic historian of the conflict, its origins ‘lay in the deliberate choice of warfare as an instrument of policy by two of the world’s most economically developed states. Far from having economic reservations about warfare as a policy, both the German and Japanese governments were influenced in their decisions for war by the conviction that war might be an instrument of economic gain.’

Milward’s judgement that economic impulsion drove Japan to war is incontestable. It was Japan’s belief that her swelling population, overflowing an island homeland deficient in almost every resource, could be supported only by taking possession of the productive regions of neighbouring China which had brought her into direct diplomatic conflict with the United States in 1937-41; it was America’s reactive trade embargoes, designed to hamstring Japan’s strategic adventurism, that in 1941 drove the Tokyo government to choose war rather than circumscribed peace as its national way forward. In the year of Pearl Harbor 40 per cent of Japan’s requirements of steel had to be imported to the home islands, together with 60 per cent of her aluminium, 80 per cent of her oil, 85 per cent of her iron ore and 100 per cent of her nickel. America’s threat to deny her oil and metals, against a guarantee of good behaviour as Washington should judge it, was therefore tantamount to strangulation. The ‘southern offensive’ was an almost predictable outcome.

Hitler could not argue economic insufficiency to justify his strategic adventurism. In 1939, when a quarter of the population was still employed on the land, Germany was almost completely self-sufficient in food, needing to import only a proportion of her consumption of eggs, fruit, vegetables and fats. She also produced all the coal she consumed and a high proportion of her iron ore, except for armaments-grade ore which was supplied from Sweden. For rubber and oil – commodities for which coal-based substitutes would be found during the war – she was wholly dependent on imports, as she was also for most non-ferrous metals. However, through peaceful trade, her high level of exports (particularly of manufactures such as chemicals and machine tools) easily earned the surplus to fund and make good those deficiencies. Had it not been for Hitler’s social-Darwinian obsession with autarky – total national economic autonomy – Germany would have had no reason to prefer military to commercial intercourse with her neighbours.

Paradoxically it was Germany’s adversaries, Britain and France, and her half-hearted ally, Italy, which had the better economic reasons for going to war. Italy was a major energy importer, while her industry, particularly her war industry, was rooted in a tradition of craftsmanship quite inconsistent with the ruthless mass consumption of the modern battlefield. Italian aero-engines were works of art – no consolation to the Regia Aeronautica pilots when replacement aircraft failed to come off the production-line at a rate to match attrition in the skies over Malta and Benghazi. France, too, maintained military arsenals run on artisan principles; and, though the country fed itself with ease and exported luxuries in plenty, it depended on its empire and its trading partners for many raw materials and some manufactured goods – for advanced aircraft, for example, from the United States in the crisis of 1940, and for rubber from its colonies in Indo-China.

Britain’s case was the most paradoxical of all. In high gear, its industry could produce all the weapons, ships, aircraft, guns and tanks that its mobilised military population could man on the battlefield. As it had demonstrated in the First and events would prove in the Second World War, moreover, it could continue to find a surplus of armaments to export (to Russia) or to re-equip exile forces (Poles, Czechs, Free French), even at the nadir of its military fortunes. However, it could do so only by importing much of its non-ferrous metal and some of its machine-tool requirements to supply its factories, all its oil, and – most critically of all for an overpopulated island – half its food. At a pinch the Japanese, by living on unhusked rice, could survive at near-starvation level. The British, if deprived of North American wheat, would in the few months it would take to exhaust the national strategic reserve of flour and powdered milk have undergone a truly Malthusian decline and halved in numbers.

Hence Winston Churchill’s heartfelt admission, once victory came, that ‘the only thing that really frightened me during the war was the U-boat peril. . . . It did not take the form of flaring battles and glittering achievements, it manifested itself through statistics, diagrams and curves unknown to the nation, incomprehensible to the public.’ The most important statistics were easily set out. In 1939 Britain needed to import 55 million tons of goods by sea to support its way of life. To do so it maintained the largest merchant fleet in the world, comprising 3000 ocean-going vessels and 1000 large coastal ships of 21 million gross-register (total capacity) tons. Some 2500 ships were at sea at any one time: the manpower of the merchant service, a resource almost as important as the ships themselves, totalled 160,000. To protect this fleet the Royal Navy deployed 220 vessels equipped with Asdic – the echo-sounding equipment developed by the Allied Submarine Detection Investigation Committee in 1917 – consisting of 165 destroyers, 35 sloops and corvettes and 20 trawlers. The ratio between merchant ships and escorts was thus about 14:1. Convoy, the practice of assembling merchant ships in organised formations under naval escort, was no longer a controversial procedure as it had been in the First World War; the Admiralty was committed to it before war broke out and it was introduced on the oceanic routes immediately and in coastal waters as soon as practicable.

U-boats and surface raiders

The principal enemy of convoy was the submarine, or U-boat (Unterseeboot). As in 1914 the Germans also deployed a number of surface commerce raiders, including both orthodox warships and converted merchantmen, but their number was small; between September 1939 and October 1942 less than a dozen auxiliary raiders gained great waters, of which the most successful, Atlantis, sank twenty-two vessels before interception and destruction by HMS Devonshire in November 1941. Germany’s battleships, battlecruisers, pocket battleships and cruisers occasionally raided the sea lanes, but they too were few in number, and judged too valuable, to be risked often, particularly after the humiliating defeat of the pocket battleship Graf Spee off Montevideo by three British cruisers in December 1939. German aircraft achieved some success as ship destroyers – in May 1941, a peak month, they sank 150,000 tons (the average displacement of a Second World War merchant ship was 5000 tons) – and mines, whether laid by aircraft, surface ship or submarine, were a constant menace. German fast coastal craft, known to the British as E-boats, were prolific minelayers in British coastal waters in 1941-4, and constituted a relentless threat to coastal convoys; in April 1944 a raid on an American troop convoy practising disembarkation for D-Day at Slapton Sands in Devon drowned more GIs than were lost off Normandy on 6 June itself. However, the attacks of aircraft and surface ships, large and small, on merchant shipping were extraneous to the real battle at sea in European waters in the Second World War. That was one, as Winston Churchill rightly denoted, between the convoy escort and the U-boats.

In September 1939 Karl Dönitz, the German U-boat admiral, had fifty-seven U-boats under command, of which thirty were short-range coastal types and twenty-seven ocean-going. The German navy’s pre-war expansion programme, the ‘Z-plan’, called for the construction of a fleet of 300, with which Dönitz claimed he could certainly strangle Britain to death. He was to achieve that total in July 1942, allowing him to maintain 140 boats on operations and sink shipping at an annual rate of 7 million tons, a figure which exceeded British building of replacement shipping more than five times. By then, however, thanks to the inescapable dynamic of warfare, almost every term in the equation by which he had calculated the inevitability of Britain’s strangulation by U-boat tactics had changed to his disadvantage. Requisition and chartering of foreign ships had added 7 million tons to the British merchant fleet, the equivalent of a year’s torpedoing. American shipyard capacity, enormously expanded by an emergency mobilisation, had been added to the British, promising an output of 1500 new ships in 1943 (including many of 10,000-15,000 tons), more than three times as many as the U-boats were sinking. Naval construction in the United States would add 200 escorts a year to the fleet between 1941 and 1945. Over 500 of these would go to join the Royal Navy’s escort fleet in the North Atlantic which, having reached a strength of 374 in March 1941, had almost doubled since the outbreak. Long-range aircraft based in North America, Iceland and Britain were progressively reducing the ‘air gap’ in which U-boats could safely operate on the surface, their preferred mode because of their low submerged speed; and integral aircraft protection for convoys, provided by ‘escort carriers’, was soon to level a direct threat against attacking U-boats. Only in facilities for basing his boats had Dönitz’s position improved; in electronic and cryptographic warfare the conflict hung in the balance; the promise of secret underwater weapons, which favoured Germany, could not be realised for some years. Nevertheless the U-boats had already inflicted severe material and psychological damage on the Allied, particularly the British, war effort; and in mid-1942 the eventual outcome of the Battle of the Atlantic was evident to no one. The ‘statistics, diagrams and curves’ were pregnant with menace.

Thus far the Battle of the Atlantic (Churchill had coined the term) had passed through four distinct phases. From the outbreak of the war until the fall of France, the U-boat fleet had been confined by geographical constraints and Hitler’s concern for the sensitivities of neutrals to operating within the immediate vicinity of the British Isles. After June 1940, when Germany gained possession of the French Atlantic ports (where in January 1941, with remarkable prescience, Hitler ordered the construction of bomb-proof U-boat ‘pens’ to begin), the fleet began to operate in the eastern Atlantic, concentrating in particular on the ‘Cape route’ to West and South Africa and occasionally penetrating into the Mediterranean, since the Italians were proving themselves inept submariners. From April to December 1941, thanks to their increasing expertise in anti-convoy tactics, and despite the delineation of an American ‘Neutrality Zone’ in which the United States Navy gave notice of its intention to attack marauding submarines, the U-boat captains began to extend their operations into the central and western Atlantic; after June 1941, when Britain began to run convoys to North Russian ports with war supplies, the U-boats, frequently supported by German warships and shore-based aircraft, also began to operate in Arctic latitudes. Finally, after December 1941, Dönitz’s men carried the submarine war to the Atlantic coast of the United States and into the Gulf of Mexico, where, during a gruesomely named ‘Happy Time’ of several months caused by the US Navy’s temporary inability to organise convoy on coastal routes, they sank coastwise shipping by hundreds of thousands of tons.

Until June 1940 the U-boats had been confined by the same facts of geography as had kept the German High Seas Fleet close to their home bases during the First World War. Using the Baltic as their training ground (as they were to do throughout the war), they attacked British shipping in the North Sea but were denied egress via the Channel by the mine barrier in the Dover Straits and could reach the Atlantic only by making the long passage round the north of Scotland – if, that is, they had the range to do so. Few had that range. Only eight of the Type IX were truly oceanic, with a range of 12,000 miles; eighteen could cruise as far as Gibraltar; the remaining thirty could not leave the North Sea. Despite these limitations the U-boats had some notable successes, including the sinking of the battleship Royal Oak in the Royal Navy’s main base at Scapa Flow in October 1939 and the aircraft carrier Courageous which was sunk while flagrantly neglecting anti-submarine precautions in home waters. From the outbreak of war to the fall of France, total merchant sinkings in the North Atlantic did not exceed 750,000 tons and 141 ships.

The capture of the French Atlantic ports in June 1940, however, transformed the basis of U-boat operations. Possession of Brest, Saint-Nazaire, La Rochelle and Lorient put Dönitz’s boats on the doorstep of Britain’s trade routes, thus ensuring that the pattern of sinkings, thitherto arbitrary and sporadic, should become regular and consistent. As soon as his crews cleared the Bay of Biscay they found themselves astride the route from Britain to the Cape along which travelled Nigerian oil and South African non-ferrous ores; and by reaching out only a little further into the Atlantic they could attack convoys carrying meat from the Argentine and grain from the United States.

Ships sailing individually were desperately vulnerable to interception. As the Royal Navy’s experience in the First World War had proved, independent sailing presented U-boats with a succession of targets: a captain who missed one lone ship on a well-used trade route could still count upon the appearance of another and thus achieve a respectable success rate by the operation of the probability factor alone. Convoy upset probability. Because the submerged speed of a submarine was at best equal to and often lower than that of a merchant ship, a U-boat captain who was wrongly positioned for an attack when a convoy hove into view would miss all the ships in it and might have to wait days before another appeared, with no greater certainty than before of finding himself correctly positioned to attack it.

Dönitz, a First World War submarine captain, had recognised the mathematical disadvantage at which his naval arm operated and conceived a method to overcome it. By experimentation with surface torpedo-boats, during the period when Germany was denied U-boats by the Versailles Treaty, he demonstrated that ‘packs’ of submarines, if disposed in a chain on the surface where their speed exceeded that of merchant ships, could identify the approach of convoys across a wide band of ocean, be concentrated against one by radio command from shore and inflict mass sinkings by a concerted raid in numbers that would overwhelm the escorts. Once Germany acquired the French Atlantic ports, it was these ‘wolf pack’ tactics that were to make the Battle of the Atlantic the knife-edge struggle for advantage which overshadowed Churchill’s conduct of the British war effort from mid-1940 to mid-1943.

Convoy, which was the Royal Navy’s defence against the wolf packs, offered only partial protection to the Atlantic lifeline. The naval escorts themselves – in the early days perhaps only two or three destroyers and a corvette were available to shepherd forty freighters and tankers across 3000 miles of ocean – were little direct threat to a determined U-boat formation. Asdic, the echo-sounder used to detect submerged U-boats, was ineffective beyond 1000 yards and reflected only range and bearing, not (until 1944) depth. The depth charges used to attack U-boats, triggered by water-pressure fuses, had to be set by guess and fractured the U-boat hull only if detonated close by. Most U-boat attacks, moreover, were delivered from the surface at night, when radar was more useful than Asdic, but until 1943 radars were too primitive to give early warning or accurate ranging.

The radio intelligence war

It was the measures taken to route convoys away from known or suspected U-boat patrol lines which best assured their safety, together with ancillary measures – particularly aerial patrol – to force U-boats to submerge while convoys passed by. Until May 1943 a shortage of aircraft and their shortness of range left an ‘air gap’ between North America, Iceland (available as a base to Britain after the German invasion of Denmark in April 1940) and Britain itself in which U-boats operated without fear of surveillance; the gap was closed when the very-long-range Liberator (B-24) with an endurance of eighteen hours came into service. Rerouteing, on the other hand, was a stratagem employed from the very beginning of the Atlantic war, and there was always a strong sense of direct conflict between the two sides. On the German side, the officers of the B-Dienst (Observer Service) used wireless intercepts and decrypts of cipher transmissions to establish convoy positions and read their orders; on the British (later Anglo-American) side, the cryptographers of the Government Code and Cipher School at Bletchley and the staff of the Admiralty Operational Intelligence Centre monitored the signals sent between U-boats and from Dönitz’s headquarters at Kernevel, Lorient (after March 1942 Berlin), to detect the formation of patrol lines and the vectoring of wolf packs against their targets. Rerouteing was by far the most successful of convoy protection measures. Between July 1942 and May 1943, for example, the British Admiralty and US Navy Department intelligence centres managed to reroute 105 out of 174 threatened North Atlantic convoys clear out of danger, and minimised attacks on another 53 by rerouteing; only 16 ran directly into wolf-pack traps and suffered heavy loss.

The success achieved by Captain Rodger Winn, RNVR, and later by his American counterpart, Commander Kenneth Knowles, depended ultimately upon the skill of the Bletchley Park cryptographers in decrypting the Kernevel U-boat traffic fast enough for its significance to be applied to convoy operations. That traffic was, of course, enciphered on the Enigma machine, and the ‘Shark’ key used by the U-boat service proved particularly resistant to Bletchley’s efforts; it was not broken until December 1942 and then not regularly until 1943. Much of the vital radio intelligence used by the Operational Intelligence Centre up to that time was of lower-grade, position-fixing quality. High Frequency Direction Finding (HF/DF or ‘Huff Duff’) enabled ships to detect and locate shadowing U-boats from the transmissions they sent back to U-boat headquarters, and so for convoys to be rerouted or protecting aircraft summoned. Meanwhile, because of the Admiralty’s ill-advised persistence in the use of a book code instead of a machine cipher, the B-Dienst was able to read convoy traffic and direct wolf packs on to chosen routes with sometimes disastrous effect.

The crux of this radio intelligence war began with the move of the U-boats from the eastern to the central Atlantic after April 1941. Substitution and rationing in Britain had allowed the import requirement to be reduced from 55 to 43 million tons, but the minimum level of subsistence was approaching and had to be measured against a rate of sinking which threatened to outstrip replacement building. In February 1941 the United States had enacted a Lend-Lease law which, in effect, allowed Britain to borrow war supplies against the promise to repay after victory; and from April 1941 the United States was operating a Neutrality Patrol which effectively excluded U-boats from the Atlantic west of Bermuda, under the terms of the Pan-American Neutrality Act of 1939. However, the U-boat fleet now had over 2000 miles of ocean in which to intercept convoys and was adding to its numbers at a considerably higher rate than it was losing U-boats on operations; during 1941 the building rate exceeded 200, while the total lost since September 1939 was less than fifty.

The eight months of extended U-boat warfare in the Atlantic in 1941 therefore proved extremely successful to the German navy. In May it suffered the loss of the great battleship Bismarck, unwisely unleashed as a commerce raider, at the end of a great chase by most of the British Home Fleet; but that defeat was offset by the sinking of 328 merchant ships of 1,500,000 tons at a time when the British yards were launching less than a million tons of new construction annually. The casualties took down with them almost every category of material of which the home islands stood in dire need – wheat, beef, butter, copper, rubber, explosives and oil, as well as military equipment.

Apologists for the British effort could show, however, that two-thirds of the ships lost had been sunk out of convoy and that U-boat losses for the year totalled 28, suggesting that the escorts’ success was increasing. Dönitz was certainly prepared to draw that conclusion: as soon as the United States Navy became an overt combatant rather than a hostile neutral (as it had been since September 1941), he transferred the weight of his effort to the United States coast. From January 1942 onwards, up to twelve U-boats were cruising off the American east coast and in the Gulf of Mexico at any one time; between January and March they sank 1.25 millon tons of shipping, equivalent to an annual rate four times higher than that achieved in the North Atlantic during 1941.

By May, however, convoy had been introduced on America’s Eastern Sea Frontier and sinkings at once declined in those waters. Moreover, the rate of new building, both of merchantmen and escorts, began to accelerate spectacularly, as the American shipyards revived and were mobilised for new construction. Of particular importance was the appearance of a standardised tanker, the T10, and a freighter, the Liberty ship, both of which were larger (14,000 and 10,000 tons) and faster than their pre-war equivalents, besides – most important of all – being quick to build. Three months was the average construction time; by October 1942 American yards were launching three Liberty ships a day and in November the Robert E. Peary was built from the keel up in four days and fifteen hours – a public-relations stunt, but grim evidence to Dönitz of the challenge that prefabrication techniques posed to the efforts of his U-boat captains.

The critical point

By July 1942, though neither side yet perceived it, the Battle of the Atlantic was approaching its crux. There were diversions from the central issue. In March 1942 the British destroyed the dock at Saint-Nazaire which offered an Atlantic coast home to theTirpitz, Germany’s largest battleship. It was then harboured in north Norway, where it had been joined in February by the Scharnhorst and Gneisenau after their daring dash through the Channel – an incident that provoked much recrimination between the Admiralty and the RAF over who bore responsibility for the failure to intercept them. The three heavy ships levelled a menace against the Arctic convoys for months to come and caused lethal disruption to convoy PQ17 in July. In December 1943 Scharnhorstmade a brief sortie against an Arctic convoy, provoking much anxiety at the Admiralty, but suffered an identical fate to that of Bismarck; and the Tirpitz menace did not end until November 1944 when she was sunk by bombing at her moorings in Troms| Fiord. But these episodes largely summarise the contribution of the German surface fleet to the Kriegsmarine’s war. The need to mount the North African ‘Torch’ landings in November 1942 temporarily drained away Allied merchantmen and warships from the Atlantic lifeline. There was a serious interruption of Ultra decrypts of U-boat radio traffic in February 1942 which lasted for most of the year, at a time when the B-Dienst was enjoying increased success against the Royal Navy’s no. 3 book code. The British, American and Canadian convoy control systems were simultaneously experiencing problems of ‘shaking down’ into a routine of co-operation; the Royal Canadian Navy, which was undergoing an expansion from six to nearly 400 warships in service, the largest of any armed force in any country during the Second World War, found particular difficulty in matching the expertise of its larger partners. Since the outbreak of the war the Admiralty and the Royal Air Force had been locked in a quarrel over the deployment of long-range aircraft, the Admiralty rightly but vainly arguing that convoy protection produced a better return of effort than spectacular but often ineffective offensive bombing of German cities. Against this background Dönitz was working to extend the range of his U-boats by experimentation with refuelling at sea from submarine ‘milch cows’, and to equip his boats for the increasingly dangerous surface transit of the Bay of Biscay against attack by such long-range aircraft as the RAF did allocate to the Admiralty via Coastal Command. In the first half of 1942 Coastal Command aircraft mounting the new and powerful Leigh searchlight began to surprise U-boats in the Bay at night and attack them with depth charges. Two were destroyed in July, although Dönitz had already ordered that they must make the passage submerged, despite the delay entailed in reaching the Atlantic cruising grounds. The significance of the Leigh Light was that it gave aircraft ‘eyes’ in the final 2000 yards of their approach when radar did not work. In a see-saw of technical duelling, its usefulness was to be reduced when the Germans learned to develop passive radar detectors which indicated danger before the Leigh Light could be activated. Over the course of the Bay of Biscay battle, however, which lasted into 1944, the advantage consistently returned to the Allies. It was not until the deployment of the first schnorkel-equipped U-boats (which could recharge batteries while cruising submerged) in early 1944 that the danger levelled by anti-submarine aircraft began to be offset.

By then, however, the climactic phase of the Atlantic battle had come and gone. From July 1942 onward, when Dönitz at last achieved his target figure of 300 U-boats, he redeployed his effort to the central Atlantic, where the Allied escort fleet had been weakened by the transfer of British ships to help the Americans introduce convoy on the Eastern Sea Frontier. He was also now becoming more adept at organising patrol lines and concentrating wolf packs against convoys, and he was having greater success in locating convoys because of the advantage in cryptography that the B-Dienst currently enjoyed over Bletchley. The British responded with two experimental measures which would bear fruit later: the creation of a ‘support group’ of escorts to go to the rescue of a convoy under attack and the adaptation of merchant ships to fly off aircraft, the merchant aircraft carriers. The MACs however, proved clumsy forerunners of the true escort carriers, the first of which, USS Bogue, would not appear until the following March; while the persisting shortage of escorts would compel the 20th Support Group to be disbanded after two months.

As a result, U-boat sinkings in the North Atlantic in November 1942 reached the total of 509,000 tons, a figure exceeded only once before in the previous May, during the ‘Happy Time’ off the American coast. Vicious Atlantic weather halved sinkings in December and January; but in February 1943, despite continuing bad weather, 120 U-boats sank nearly 300,000 tons in the North Atlantic and the toll seemed set to rise. During March, in a running battle against two convoys eastbound from North America to Britain, codenamed HX229 and SC122, forty U-boats sank twenty-two out of ninety merchantmen and one escort of the twenty which were defending them. The tonnage sunk, 146,000 was the highest in any convoy battle and led Dönitz and his crews to believe that they had victory in their grasp. Altogether they sank 108 ships in the North Atlantic in March, totalling 476,000 tons, the majority lost in convoy. Wolf-pack tactics, supported by the position-finding and decrypting successes of the B-Dienst, appeared to have achieved mastery over convoy protection.

The appearance was illusory. Not only was the shipping replacement rate rising to meet losses (by October 1943 new construction had actually made good the amount of shipping lost since 1939, and built a superior merchant fleet into the bargain); U-boat losses were also beginning to equal launchings, at a monthly rate of about fifteen. Those statistics spelt doom to Dönitz’s effort. The explanation of the shift lay in several directions: Bletchley recovered its edge over the B-Dienst in May 1943, making rerouteing even more successful; escorts were becoming more plentiful, allowing the creation of permanent ‘support groups’, five in number in April; to the existing escorts were added two escort carriers embarking twenty anti-submarine aircraft which could force all U-boats in the vicinity of a convoy to submerge, thus effectively negating their offensive potential; improved radar, Asdic and depth-charge launchers (Hedgehog and Squid) levelled a direct tactical threat against the U-boat in close combat; but, above all, the increased availability of long-range patrol aircraft for the Atlantic battle wrought a strategic shift of advantage to the British-US-Canadian side. The long-range aircraft, particularly the Liberator bomber, equipped with radar, Leigh Light, machine-guns and depth charges, was flying death to a surfaced U-boat. In the Bay of Biscay – after a short and disastrous episode of ‘fighting it out’, ordered by Dönitz – the aircraft forced all U-boats to make their passage to the North Atlantic hunting-grounds submerged, at a fourfold time penalty; in great waters they completely disrupted Dönitz’s wolf-pack tactics by dispersing his patrol lines and savaging his concentrations wherever they appeared. In May 1943 U-boat losses, inflicted at a ratio of about 3:2 between aircraft and escorts, reached forty-three, which exceeded replacement more than twice. On 24 May Dönitz, accepting the inevitable, withdrew his fleet from the ocean, conceding later in his memoirs: ‘We had lost the Battle of the Atlantic.’

That did not mark the end of the U-boat war. In May 1944 the first schnorkel-equipped U-boat made its trial cruise. The schnorkel, a retractable air-breathing tube, allowed a submarine to cruise submerged while using its diesel engines. The device, invented by a Dutch naval officer in 1927, anticipated the development both of the closed hydrogen-peroxide system, which the Germans would bring into service in 1945, and of nuclear propulsion, in that it transformed the submersible U-boat into a true submarine, capable of operating below the surface throughout an operational mission. Misused, it could kill a crew by suffocation, and two U-boat crews are believed to have died in that way; properly employed, it would have revived the U-boat threat. Had the Germans not lost their main Atlantic ports to the American army in August 1944, schnorkel U-boats would have reopened the Atlantic battle, to the Allies’ very great cost.

Measured across the space of the Atlantic struggle, from September 1939 to May 1943, the cost can be seen to have fallen most heavily on Dönitz’s submarine arm. Although the Allies lost 2452 merchant ships in the Atlantic, of nearly 13 million gross-register tons, and 175 warships, mostly British (a term which included Canadian, Polish, Belgian, Norwegian and Free French escorts also), the Kriegsmarine lost 696 out of 830 U-boats dispatched on operations, almost all in the Atlantic, and 25,870 killed out of 40,900 crewmen who sailed; another 5000, plucked from the wrecks of their depth-charged boats, became prisoners. This casualty rate – 63 per cent fatal, 75 per cent overall – far exceeded that suffered by any other arm of service in the navy, army or air force of any combatant country.

The cost was certainly not in vain. Given that the economic odds disfavoured Germany from the outset, that its industry was organised ‘in breadth’ for a short war rather than ‘in depth’ for a long war, and that Hitler’s campaign of conquest was notably unsuccessful in adding either productive capacity or raw material resources to the Reich’s war-making capacity – it failed, for example, to acquire any large source of oil or non-ferrous metal ores for the German war machine – the delaying effect of the U-boat war on the transformation of Britain into an Anglo-American place d’armes for the eventual liberation of Europe may be seen as crucial. Moreover, while Germany fed itself easily between 1940 and 1944 from its own agricultural output and the requisitions made on farming in the occupied lands to the east and west, Britain was constantly held close to the level of minimum subsistence by U-boat depredations on its food imports. Rationing, though fairly applied and beneficial to the classes nutritionally deprived before the war, created a climate of latent crisis among the British which distorted and diminished their capacity to strike back at the enemy. Britain’s military threat to Germany during the Second World War was as intense as that levelled during the First though in relative terms Britain was not much weaker in 1940-4 than in 1914-18. It was the U-boats, marginally assisted by the Luftwaffe, which made the difference.

The U-boats were also to prove of crucial significance in diffusing and diminishing the support brought by British and especially American industry to their allies and their own ancillary theatres of war. Russian industry was devastated by the German invasion of White Russia and the Ukraine in 1941, and the Soviet Union’s capacity to sustain resistance was only salved by the almost incredibly swift transfer of factories from the western provinces to the trans-Ural regions in the terrible winter of 1941-2. Between July and October, for example, 496 factories were transported by train from Moscow to the east, leaving only 21,000 out of 75,000 metal-cutting lathes in the capital; overall the Russian railways moved 1523 factories from west to east between June and August, and between August and October it was calculated that 80 per cent of Russian war industry was ‘on wheels’, moving from the threatened zones to areas of safety in western or eastern Siberia. The disruption of production entailed by this unprecedented industrial migration could only be made good by substitutions from Western sources, of weapons and munitions but above all of the elements of war’s infrastructure – vehicles, locomotives and rolling stock, fuel, rations and even such simple but vital supplies as boots, the felt winter boots for lack of which tens of thousands of German soldiers lost toes in the winter of 1941-2. Between March 1941 and October 1945 the United States supplied the Soviet Union with 2000 locomotives, 11,000 rail wagons, nearly 3 million tons of gasoline, 540,000 tons of rails, 51,000 jeeps, 375,000 trucks and 15,000,000 pairs of boots. It was in American boots and trucks that the Red Army advanced to Berlin. Without them its campaign would have foundered to a halt in western Russia in 1944.

Boots and trucks proved far more important items of war supply than the 15,000 aircraft, 7000 tanks and 350,000 tons of explosives which Lend-Lease also consigned to the Soviet Union; far more important than all the aid sent by Britain during the course of the war – 5000 tanks, 7000 aircraft, even the 114,000 tons of rubber. Vital though these war supplies were, however, they reached Russia between 1941 and 1944 by the most circuitously inconvenient routes, thanks to Dönitz’s U-boat campaigns. The ‘North Russia run’ from Britain to Murmansk and Archangel had to be routed almost as far west as Greenland and as far north as Spitsbergen (on which a strange little sub-war for possession of weather stations was fought in 1941-2) during the summer months of 1941-4, to avoid air and sea attack by German units based in Norway; when the winter ice drove the convoys eastward, losses rose grievously, forcing Churchill to interrupt sailings on several occasions – to Stalin’s woundingly expressed scorn. The alternative route through the Persian Gulf was roundabout and terminated at the railhead of a long and inadequate railway system. The Pacific route, to Vladivostok, was also affected by ice and the danger of enemy attack, and it connected with the wrong end of the longest railway line in the world, the Trans-Siberian.

Hitler’s investment in his U-boat fleet thus more than justified the cost. It exerted a partial strangulation on the offensive effort of the immediate enemies, the Russians most of all but also the British, delayed the build-up of a large American expeditionary force on his doorstep and hampered the development of a hostile ‘peripheral’ strategy in the Mediterranean; the closing of that sea to regular British convoying in 1940-2, largely through aerial but partly through submarine threat, forced the desert army to depend on supply via the Cape route, 12,000 miles long, at very great cost to its efficiency.

Had Hitler achieved the creation of a 300-boat fleet before 1942, added significantly to its size thereafter, or managed to introduce his advanced schnorkel and revolutionary hydrogen-peroxide types before 1944, partial might have become total strangulation. None the less, Germany considerably maximised the advantage of being able to operate from the centre of its strategic area – the advantage of ‘interior lines’ that continental powers had traditionally exercised against oceanic enemies in the European world. Dönitz proved by far the most useful of all the military subordinates who lent their services to Hitler’s campaign of conquest – far more useful than Goering, the Luftwaffe chief, or even von Braun, the father of his pilotless missiles – and it was entirely appropriate that he should have been nominated to succeed him as Führer in the last days of the Reich. In pitiless and self-immolatory dedication to the creed of total war, Nazism found no equal within the Wehrmacht to the U-boat arm. Its ‘aces’ – Günther Prien, Otto Kretschmer, Manfred Kinzel, Joachim Schepke – whether believing Nazis or not, personified its ethos of the superman and even succeeded, for all the cruelty they inflicted, in winning the respect of their enemies for their warrior prowess. The British officer who interrogated Kinzel, ‘ace of aces’ with a slate of 270,000 tons of shipping sunk, ruefully expressed the hope that ‘there were not too many like him’.

The Atlantic was not the only via dolorosa of war supply. The Burma Road and ‘the Hump’, over which supplies were driven or flown (at 14,000 feet) to Chiang Kai-shek’s army in south China, were others. The Takeradi route, from West to East Africa, provided the desert air force with aircraft disembarked from Atlantic convoys and assembled ashore. The Lake Ladoga ‘ice road’ saved Leningrad from total starvation in the winters of 1941-3. And ultimately the Japanese, who had succeeded in turning the archipelagos within their Pacific ‘island perimeter’ into the foundation of a watery ‘continental’ strategy during 1942, achieved extraordinary feats of maritime supply in keeping their far-flung garrisons combatant during 1943-4, when MacArthur’s ‘island-hopping’ plan cracked the carapace of their oceanic fortress. In 1945, at the end of their own ‘Battle of the Pacific’, when they lost almost their whole merchant fleet to America’s submarines, the home islands were on the brink of starvation by midsummer – a whimpering end to Japan’s campaign of conquest which was stopped short by the cosmic bang of Hiroshima and Nagasaki.

However, none of these logistic efforts compared in duration, magnitude and importance with the Battle of the Atlantic. It was truly both a battle and a war-winning enterprise. Had it been lost, had those ‘statistics, diagrams and curves’ which blighted Winston Churchill’s days and nights in 1940-2 turned wrong, had each U-boat on its patrol line succeeded in sinking only one more merchant ship in the summer of 1942, when losses already exceeded launchings by 10 per cent, the course, perhaps even the outcome, of the Second World War would have been entirely otherwise. The 30,000 men of the British Merchant Navy (one-fifth of its pre-war strength) who fell victim to the U-boats between 1939 and 1945, the majority drowned or killed by exposure on the cruel North Atlantic sea, were quite as certainly front-line warriors as the guardsmen and fighter pilots to whom they ferried the necessities of combat. Neither they nor their American, Dutch, Norwegian or Greek fellow mariners wore uniform and few have any memorial. They stood nevertheless between the Wehrmacht and the domination of the world.

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