The 'Battle of the Atlantic' was the longest continuous campaign of World War II and pitted Allied merchant and naval mariners and airmen against German and Italian submariners, warship personnel and aircraft crews (3 September 1939/8 May 1945).

At heart of the battle was the Allied naval blockade of Germany, announced on the day after the British and French declarations of war on Germany, and Germany’s subsequent counter-blockade. The campaign reached its peak in the period between the middle of 1940 and the end of 1943.

The 'Battle of the Atlantic' pitted U-boats and other warships of the Kriegsmarine and aircraft of the Luftwaffe against the Royal Navy, Royal Canadian Navy, US Navy, and Allied merchant shipping, which were supported by increasing fleets of warships and aircraft. Coming primarily from North America and travelling predominantly to the UK and the USSR, were protected for the most part by the British and Canadian navies and air forces. These forces were aided by ships and aircraft of the US Navy from 13 September 1941. The Germans were joined by submarines of the Regia Marina after Italy, Germany’s Axis ally, entered the war on 10 June 1940.

As an island country, the UK was highly dependent on imported goods. The UK required more than one million tons of imported material per week in order to survive and fight. In essence, the 'Battle of the Atlantic' involved a tonnage war: the Allied struggle to supply the UK and the Axis attempt to stem the flow of merchant shipping that enabled the UK to keep fighting. From 1942 onward the Axis also sought to prevent the build-up of Allied supplies and equipment in the British Isles in preparation for the invasion of occupied Europe by British, Canadian and US land forces, and the defeat of the U-boat threat was therefore a prerequisite for pushing back the Axis in Western Europe. The outcome of the battle was a strategic victory for the Allies (the German blockade failed) but at great cost: 3,500 merchant ships and 175 warships were sunk in the Atlantic Ocean for the loss of 783 U-boats (the majority of them 'Type VII' submarines) and 47 German surface warships, including four capital ships (the battleships Bismarck and Tirpitz, and the battle-cruisers Scharnhorst and Gneisenau), nine cruisers, seven raiders, and 27 destroyers. Of the U-boats, 519 were sunk by British, Canadian or other allied forces, while 175 were destroyed by US forces; 15 were destroyed by the Soviets and 73 were scuttled by their crews before the end of the war for various reasons.

The Battle of the Atlantic has been called the 'longest, largest, and most complex' naval battle in history. The campaign started immediately after the start of the war in Europe, during the so-called 'Phoney War', and lasted more than five years until the German surrender in May 1945. It involved thousands of ships in more than 100 convoy battles and perhaps 1,000 single-ship encounters, in a theatre covering millions of square miles of ocean. The situation changed constantly, with one side or the other gaining advantage, as participating countries surrendered, joined and even changed sides in the war, and as new weapons, tactics, counter-measures and equipment were developed by each side. The Allies gradually gained the upper hand, overcoming German surface raiders by the end of 1942 and defeating the U-boats by the middle of 1943, although losses due to U-boats continued until the war’s end. The British prime minister, Winston Churchill, later wrote that 'The only thing that really frightened me during the war was the U-boat peril. I was even more anxious about this battle than I had been about the glorious air fight called the ''Battle of Britain''.'

On 5 March 1941, the First Lord of the Admiralty, A. V. Alexander, asked parliament for 'many more ships and great numbers of men' to fight the 'Battle of the Atlantic', which he compared to the 'Battle of France', fought during the previous summer. The first meeting of the cabinet’s 'Battle of the Atlantic Committee' met on 19 March. Churchill claimed to have coined the phrase 'Battle of the Atlantic' shortly before Alexander’s speech, but there are several examples of the phrase’s earlier use.

Following the use of unrestricted submarine warfare by Germany in World War I, many countries tried to limit or abolish submarines. The effort failed, and instead the London Naval Treaty required submarines to abide by 'cruiser rules', which demanded they surface, search ships and place their crews in 'a place of safety' (for which lifeboats did not qualify, except under particular circumstances) before sinking them, unless the ship in question showed any 'persistent refusal to stop…or active resistance to visit or search'. These regulations did not prohibit the defensive arming of merchant vessels, but doing so, or having them report contact with submarines or raiders, made them de facto naval auxiliaries and removed the protection of the cruiser rules. This made restrictions on submarines effectively moot.

In 1939, the Kriegsmarine lacked the strength to challenge the combined British and French warships of the Royal Navy and Marine Nationale for command of the sea. Instead, German naval strategy relied on commerce raiding using capital ships, armed merchant cruisers, U-boats and aircraft. Many German warships were already at sea when war was declared in September 1939, including most of the available U-boats and the 'pocket battleships' Deutschland and Admiral Graf Spee, which had sortied into the Atlantic during August. These ships immediately started to attack British and French shipping. U-30 sank the liner Athenia within hours of the declaration of war, although this was a breach of her orders not to sink passenger ships. The U-boat fleet, which was to dominate so much of the 'Battle of the Atlantic', was very few in number at the beginning of the war: many of the 57 available boats were of the small and short-ranged 'Type II' class, useful primarily for minelaying and operations in coastal waters rather than operations in any oceanic context. Much of the early German anti-ship activity involved minelaying by destroyers, aircraft and U-boats off British ports.

On the outbreak of war, the British and French immediately began a blockade of Germany, although this had little immediate effect on German industry. The Royal Navy quickly introduced a convoy system for the protection of trade, and this system was gradually extended out from the British Isles, eventually reaching as far as Panama, Bombay and Singapore. When the convoy system was first introduced however, the Admiralty strongly opposed the idea. Britain believed that the convoy would be a waste of warships that might be needed in battle. Convoys allowed the Royal Navy to concentrate its escorts near the one place the U-boats were guaranteed to be found, namely the convoys. Each convoy consisted of between 30 and 70 mostly unarmed merchant ships.

Some British naval officials, particularly the First Lord of the Admiralty, Winston Churchill, sought a more offensive strategy. The Royal Navy formed anti-submarine hunting groups based on aircraft carriers to patrol the shipping lanes in the Western Approaches and hunt for U-boats. This strategy was deeply flawed as a U-boat, with its tiny silhouette, was always likely to spot the surface warships and submerge long before it could itself be sighted. Carrierborne aircraft were little help, for though they could spot submarines on the surface, at this stage of the war they had no weapons adequate tp deliver an attack on any such boat caught on the surface, and any boat found by an aeroplane was long gone by the time surface warships arrived. The hunting group strategy proved a disaster within days. On 14 September 1939, the UK’s most modern carrier, Ark Royal, narrowly avoided being sunk when three torpedoes from U-39 detonated prematurely. U-39 was forced to surface and scuttle by the escorting destroyers, becoming the first U-boat loss of the war. Another carrier, Courageous, was sunk three days later by U-29.

The use of escort destroyers hunting for U-boats continued to be a prominent, though misguided, tactic of British anti-submarine operations for the first year of the war. U-boats nearly always proved elusive, and the convoys, denuded of cover, were put at even greater risk.

German success in sinking Courageous was surpassed a month later when U-47 penetrated the British fleet base at Scapa Flow in the Orkney islands group and sank the old battleship Royal Oak at anchor.

In the South Atlantic, British forces were stretched by the cruise of Admiral Graf Spee, which sank nine merchant ships of 50,000 tons in the South Atlantic and Indian Ocean during the first three months of the war. The British and French formed a series of hunting groups including three battle-cruisers, three aircraft carriers and 15 cruisers to seek the raider and her sister Deutschland, which was operating in the North Atlantic. These hunting groups had no success until Admiral Graf Spee was caught off the mouth of the Plate river between Argentina and Uruguay by an inferior British force. After suffering damage in the subsequent action, she took shelter in neutral Montevideo harbour and was scuttled on 17 December 1939.

After this initial burst of activity, the Atlantic campaign became quieter. Konteradmiral Karl Dönitz, commander of the U-boat force, had planned a maximum submarine effort for the first month of the war, with almost all available U-boats out on patrol in September. That level of deployment could not be sustained, however, for the boats needed to return to harbour to refuel, re-arm, take on supplies, and refit. The harsh winter of 1939/40, which froze the waters of many of the Baltic Sea ports, seriously hampered the German offensive by trapping several new U-boats in the ice. Adolf Hitler’s plans to invade Norway and Denmark in the spring of 1940 led to the withdrawal of the fleet’s surface warships and most of the ocean-going U-boats for fleet operations in this 'Weserübung'.

The resulting Norwegian campaign revealed serious flaws in the magnetic influence pistol (firing mechanism) of the U-boats' principal weapon, the torpedo. Although the narrow fjords gave U-boats little room for manoeuvre, the concentration of British warships, troopships and supply ships provided many opportunities for the U-boats to attack. Time and again, U-boat captains tracked British targets and fired, only to watch the ships continue unharmed as the torpedoes exploded prematurely as a result of the faulty influence pistol, or hit and failed to detonate as a result of the faulty contact pistol, or ran beneath the target without exploding as a result of the incorrect functioning of the influence feature or depth control mechanism. No British warship was sunk by a U-boat in more than 20 attacks. As the news spread through the U-boat fleet, it began to undermine morale. The director in charge of torpedo development continued to claim it was the crews' fault. Early in 1941, the problems were determined to be the result of differences in the earth’s magnetic fields at high latitudes and a slow leakage of high-pressure air from the U-boat into the torpedo’s depth regulation gear. These problems had been solved by about March 1941, making the torpedo a formidable weapon. (Similar problems plagued the US Navy’s Mk 14 torpedo: the USA ignored reports of German problems.)

Early in the war, Dönitz submitted a memorandum to Grossadmiral Erich Raeder, the German navy’s commander-in-chief, in which he estimated effective submarine warfare could bring the UK to its knees because of the country’s dependence on overseas commerce. He advocated a system known as the Rudeltaktik (the so-called 'wolf pack'), in which U-boats would spread out in a long line at right angles to the projected course of a convoy. After a boat had reported a contact, the other boats converged on the reporting boat to attack en masse and overwhelm any escorting warships. While escorts chased individual boats, the rest of the 'wolf pack' would be able to attack the merchant ships with impunity. Dönitz calculated that a force of 300 of the latest 'Type VII' oceanic boats would create enough havoc among Allied shipping that the UK would be forced out of the war.

This tactic contrasted starkly with the traditional view of submarine tactics up to that time, in which the submarine was seen as a lone ambusher, waiting outside an enemy port to attack ships entering and leaving. This had been a very successful tactic used by British submarines in the Baltic Sea and Dardanelles during World War I, but it could not be successful if the port approaches were well patrolled. There had also been naval theorists who held that submarines should be attached to a fleet and used like destroyers: this had been tried by the Germans in the Battle of Jutland in 1916 and provided only poor results as underwater communications were in their infancy. The Japanese also adhered to the idea of the fleet submarine to scout ahead of the battle fleet, and never used their submarines either for close blockade or convoy interdiction. The submarine was still seen by much of the naval world as 'dishonourable', compared to the prestige attached to capital ships. This was true in the Kriegsmarine: Raeder successfully lobbied for money to be spent on capital ships rather than U-boats.

The Royal Navy’s main anti-submarine weapon before the war was the inshore patrol craft, which was fitted with hydrophones and armed with a small gun and depth charges. Like most, the Royal Navy had not considered anti-submarine warfare as a tactical subject during the 1920s and 1930s. Unrestricted submarine warfare had been outlawed by the London Naval Treaty, anti-submarine warfare was seen as 'defensive' rather than dashing, many naval officers believed that anti-submarine work was drudgery similar to minesweeping, and ASDIC submarine-detection equipment was believed to have rendered submarines impotent. Although destroyers also carried depth charges, it was expected these ships would be used in fleet actions rather than coastal patrol, so their crews were not extensively trained in the use of depth charges. The British, however, ignored the fact that arming merchantmen, as the UK did from the start of the war, removed them from the protection of the 'cruiser rules', and the fact that anti-submarine trials with ASDIC had been conducted under ideal rather than typical conditions.

The German occupation of Norway in April 1940, the rapid conquest of the Low Countries and France in May and June of the same year, and the Italian entry into the war on the Axis side in June transformed the nature of the war at sea in general and the Atlantic campaign in particular in three main ways.

Firstly, the UK lost its major ally. In 1940, the French navy was the fourth largest in the world, but only a handful of the French ships joined the Free French forces and fought against Germany after the fall of France, although these were later joined by a few Canadian destroyers. With the French fleet removed from the campaign, the Royal Navy was stretched even further. Italy’s declaration of war meant that Britain also had to reinforce its Mediterranean Fleet and establish a new group at Gibraltar, known as Force 'H', to replace the French fleet in the western basin of the Mediterranean basin.

Secondly, the U-boats now possessed direct access to the Atlantic. Since the English Channel was relatively shallow, and had been partially blocked with minefields by the middle of 1940, U-boats were ordered not to negotiate it and instead passage round the British Isles to reach the most profitable spot to hunt ships. The French Atlantic coast bases at Brest, Lorient and La Pallice were now available to the Germans and lay about 450 miles (725 km) closer to the Atlantic than Germany’s North Sea bases. This greatly improved the situation for U-boats in the Atlantic, enabling them to attack convoys farther to the west and enabling them spend longer times on patrol, doubling the effective size of the U-boat force. The Germans later built massive fortified concrete pens for the U-boats in the French Atlantic bases: these were impervious to Allied bombing until the middle of 1944, when the Tallboy bomb became available. From a time early in July 1940, U-boats returned to the new French bases after completing their Atlantic patrols.

Thirdly, British destroyers were diverted from the Atlantic. The Norwegian campaign and the German invasion of the Low Countries and France imposed a heavy strain on the Royal Navy’s destroyer flotillas. Many older destroyers were withdrawn from convoy routes to support the Norwegian campaign in April and May and then diverted to the English Channel to support the withdrawal from Dunkirk. By the summer of 1940, the UK then faced a serious threat of invasion. Many destroyers were held available for operations in the English Channel to repel any German invasion. They suffered heavily under air attack by the Luftwaffe. Seven destroyers were lost in the Norwegian campaign, another six in the 'Battle of Dunkirk' and 10 more in the Channel and North Sea between May and July, many to air attack as they lacked adequate anti-aircraft armament. Dozens of other destroyers were damaged.

The completion of Hitler’s campaign in Western Europe meant that U-boats withdrawn from the Atlantic for the Norwegian campaign now returned to the war on British trade. So at the very time the number of U-boats on patrol in the Atlantic began to increase, the number of escorts available for the protection of convoys was greatly reduced. The only consolation for the British was that the large merchant fleets of occupied countries such as Norway and the Netherlands came under British control. After the German occupation of Denmark and Norway, British forces occupied Iceland and the Faroe islands group, establishing bases there and preventing a German takeover.

It was in these circumstances that Churchill, who had become prime minister on 10 May 1940, first wrote to President Franklin D. Roosevelt of the USA to request the loan of 50 obsolescent US Navy destroyers. This eventually led to the 'Destroyers for Bases Agreement' (effectively a sale but portrayed as a loan for political reasons), by which the destroyers were made available to the UK in exchange for 99-year leases on certain British bases in Newfoundland, Bermuda and the West Indies: this was a financially advantageous bargain for the USA but militarily beneficial for the UK as it effectively freed British military assets to return to Europe. A significant percentage of the US population opposed entering the war, and some US politicians believed that the UK and its allies might lose the war. The first of the 50 destroyers were taken over by their British and Canadian crews only in September, and all needed to be rearmed and fitted with ASDIC, so it was to be many months before the ships contributed to the campaign.

The early U-boat operations from the French bases were very successful. This was the heyday of the great U-boat aces such as Günther Prien of U-47, Otto Kretschmer of U-99, Joachim Schepke of U-100, Engelbert Endrass of U-46, Victor Oehrn of U-37 and Heinrich Bleichrodt of U-48. U-boat crews became heroes in Germany. Between June and October 1940, more than 270 Allied ships were sunk, and the this became known to U-boat crews as their glückliche Zeit (happy time).

The U-boats' major challenge was to find the convoys in the vastness of the ocean. The Germans had a handful of very long-range Focke-Wulf Fw 200 Condor four-engined aircraft based at Bordeaux and Stavanger, and these were used for reconnaissance. The Condor was a converted airliner, and seen as a 'stopgap' solution for the requirement of the Fliegerführer 'Atlantik'. As a result of continuing friction between the Luftwaffe and Kriegsmarine, however, the primary source of convoy sightings was the U-boats themselves. Since its bridge was low to the water, the U-boat’s range of visual detection was quite limited.

The other best source of information about British convoy movements proved to be the codebreakers of the B-Dienst, the German naval radio interception service, which had succeeded in deciphering the British Naval Cypher No. 3, allowing the Germans to estimate where and when convoys could be expected.

In response, the British applied the techniques of operational research to the problem and came up with some counter-intuitive solutions for protecting convoys. They realised that the area of a convoy increased by the square of its perimeter, meaning the same number of ships, using the same number of escorts, was better protected in one convoy than in two, and that one large convoy was therefore as difficult to locate as single small convoy. Moreover, reduced frequency also reduced the chances of detection, as fewer large convoys could carry the same amount of cargo as several smaller convoys, although large convoys took longer to assemble. Therefore, a few large convoys with apparently few escorts were safer than many small convoys with a higher ratio of escorts to merchantmen.

Instead of attacking the Allied convoys singly, U-boats were directed to work in wolf packs co-ordinated by radio. The boats spread out into an extended patrol line, at intervals of about twice the visual horizon of a single boat, that bisected the path of the Allied convoy routes. Once in position, the crew studied the horizon through binoculars looking for masts or smoke, or used hydrophones to pick up propeller noises. When one boat sighted a convoy, the fact was reported to U-boat headquarters, the boat shadowing and continuing to report as needed until the arrival of other boats, ordered to the area by radio, typically at night. Instead of being faced by a single boat, therefore, the convoy escorts had to cope with groups of up to half a dozen boats attacking simultaneously. The most daring commanders penetrated the escort screen and attacked from within the columns of merchantmen. The escort vessels, which were too few in number and often lacking in endurance, had no answer to several boats attacking on the surface at night as their ASDIC only worked well against underwater targets. Early British maritime radar, working in the metric bands, lacked target discrimination and range. Moreover, the corvettes constituting the majority of the escorts, were too slow to catch a surfaced boat.

The wolf pack tactic was first used successfully in September and October 1940 to devastating effect in a series of convoy battles. On 21 September 21, the HX.72 convoy of 42 merchant vessels was attacked by a pack of four U-boats, which sank 11 ships and damaged two others over the course of two nights. In October, the SC.7 slow convoy, escorted by two sloops and two corvettes, was overwhelmed, losing 59% of its ships. The battle for the HX.79 convoy in the following days was in many ways worse for the escorts than for the convoy. The loss of a quarter of the convoy without any loss to the U-boats, despite the very strong escort of two destroyers, four corvettes, three trawlers and one minesweeper demonstrated the effectiveness of the German tactic against the inadequate British anti-submarine methods of the time. On 1 December, seven German and three Italian submarines caught the HX.90 convoy, sinking 10 ships and damaging three others. The success of wolf pack tactics against these convoys encouraged Dönitz to adopt the wolf pack as his primary tactic.

At the end of 1940, the Admiralty viewed the number of ships sunk with growing alarm. Damaged ships might survive but could be out of commission for long periods. Two million gross tons of merchant shipping, representing 13% of the fleet available to the British, were under or awaiting repair and therefore unavailable, which had the same effect in slowing the flow of supplies across the Atlantic.

Moreover, the U-boats were not the only threat. Following some early experience in support of the war at sea during 'Weserübung', the Luftwaffe began to take a toll of merchant ships. Generalleutnant Martin Harlinghausen and his recently established Fliegerführer Atlantik command contributed small numbers of aircraft to the 'Battle of the Atlantic' from 1941. These aircraft were primarily Fw 200 and, later, Junkers u 290 four-engined machines, used for long-range reconnaissance. The Fw 200 machines also bombed convoys that were beyond land-based fighter cover and thus defenceless. Initially, the Fw 200 machines were very successful, claiming 365,000 tons of shipping early in 1941. These aircraft were few in number, however, and directly under Luftwaffe rather than Kriegsmarine operational control. In addition, the pilots had little specialised training for anti-shipping warfare, which limited their utility.

The Germans received help from their allies. From August 1940, a flotilla of 27 Italian submarines operated from the BETASOM base in Bordeaux to attack Allied shipping in the Atlantic, initially under the command of Ammiraglio di Divisione Angelo Parona, then of Ammiraglio di Divisione Romolo Polacchini and finally of Capitano di Vascello Enzo Grossi. The Italian submarines had been designed to operate in tasks different from those of the U-boats, and they had a number of flaws that had to be corrected (for example huge conning towers, slow speed on the surface, and lack of modern torpedo fire-control system), which meant that they were ill-suited for convoy attacks, and performed better when hunting down isolated merchantmen on distant seas, taking advantage of their superior range and living standards. While initial operations met with little success (only 65,343 tons sunk between August and December 1940), the situation gradually improved, and up to August 1943 the 32 Italian submarines which operated in the Atlantic sank 109 ships of 593,864 tons in exchange for the loss of 17 boat, giving them a submarine/tonnage sunk ratio similar to that of the German boats in the same period, and higher overall. The Italians were also successful with their use of the maiali (pig) human torpedo, disabling several British ships in Gibraltar.

Despite these successes, the Italian intervention was not seen favourably by Dönitz, who characterised Italians as 'inadequately disciplined' and 'unable to remain calm in the face of the enemy'. The Italians were unable to co-operate in wolf pack tactics or even reliably report contacts or weather conditions, and their area of operations was moved away from those of the Germans.

Among the more successful Italian submarine commanders who operated in the Atlantic were Carlo Fecia di Cossato, commander of the submarine Enrico Tazzoli, and Gianfranco Gazzana-Priaroggia, commander of Archimede and then of Leonardo da Vinci.

ASDIC, which was also known to the US Navy as sonar, was a central feature of the 'Battle of the Atlantic'. One crucial development was the integration of ASDIC with a plotting table and weapons (depth charges and later Hedgehog) to make an anti-submarine warfare system. ASDIC produced accurate data on the range and bearing of the target, but could be fooled by thermoclines, currents or eddies, and schools of fish, and therefore required experienced operators to be effective. ASDIC was effective only at low speeds: at greater tan 15 kt or so, the noise of the ship’s passage through the water drowned the echoes.

The early wartime Royal Navy procedure was to sweep the ASDIC in an arc from one side of the escort’s course to the other, stopping the transducer every few degrees to send out a signal. Several ships searching together would be used in a line, 1 to 1.5 miles (1.6 to 2.4 km) apart. If an echo was detected, and if the operator identified it as a U-boat, the escort was directed toward the target and closed at moderate speed; the U-boat’s range and bearing were plotted over time to determine course and speed as the attacker closed to within 1,000 yards (915 m). Once it had been to attack, the escort increased speed, using the target’s course and speed data to adjust her own course. The intention was to pass over the U-boat, depth charges from chutes at the stern at even intervals, while throwers fired other charges some 40 yards (37 m) to either side. The intention was to lay a 'pattern' like an elongated diamond with the submarine somewhere inside it. To disable a U-boat, a depth charge had to detonate within about 20 ft (6.1 m) of it target. Since early ASDIC equipment was poor at determining depth, it was usual to vary the detonation depth settings on part of the pattern.

There were disadvantages to the early versions of this system. Exercises in anti-submarine warfare had been restricted to one or two destroyers hunting a single submarine whose starting position was known, and working in daylight and calm weather. U-boats could dive far deeper than British or US submarines (more than 700 ft/210 m), well below the 350-ft (110-m) maximum depth charge setting of British depth charges. More importantly, early ASDIC sets could not look directly downward, so the operator lost contact with the target during the final stages of the attack, a time when the boat would certainly be manoeuvring rapidly. The explosion of a depth charge also disturbed the water, so ASDIC contact was very difficult to regain if the first attack failed, and made it possible for the target to change position with impunity.

The belief that ASDIC had solved the submarine problem, the acute budgetary pressures of the 'Great Depression', and the pressing demands for many other types of rearmament meant little was spent on anti-submarine ships or weapons. Most British naval spending, and many of the best officers, went into the battle fleet. Critically, the British expected, as in World War I, that U-boats would be coastal craft and therefore threaten only harbour approaches. As a result, the Royal Navy entered World War II without sufficient long-range escorts to protect ocean-going shipping, and there were no officers with experience of long-range anti-submarine warfare. The situation in the Royal Air Force’s Coastal Command was even more dire: patrol aircraft lacked the range to cover the North Atlantic and could typically machine gun only the spot where a U-boat had been seen to dive.

Despite its early success, the U-boat was still not recognised as the foremost threat to the North Atlantic convoys. With the exception of men such as Dönitz, most naval officers on both sides regarded surface warships as the ultimate commerce destroyers. For the first half of 1940, there were no German surface raiders in the Atlantic because the German fleet had been concentrated for the invasion of Norway. The sole pocket 'pocket battleship' raider, Admiral Graf Spee, had been stopped in the 'Battle of the River Plate' by an inferior and outgunned British squadron. From the summer of 1940 a small but steady stream of warships and armed merchant raiders set sail from Germany for the Atlantic.

The power of a raider against a convoy was demonstrated by the fate of the HX.84 convoy, which was attacked by the 'pocket battleship' Admiral Scheer on 5 November 1940. Admiral Scheer quickly sank five ships and damaged several others as the convoy scattered. Only the sacrifice of the escorting armed merchant cruiserJervis Bay and failing light allowed the other merchant vessels to escape. The British now suspended North Atlantic convoys and the Home Fleet put to sea to try to intercept Admiral Scheer. The search failed and Admiral Scheer disappeared into the South Atlantic and then reappeared in the Indian Ocean during the following month.

Other German surface raiders now began to make their presence felt. On 25 December 1940, the heavy cruiser Admiral Hipper attacked the WS.5A troop convoy, but was driven off by the convoy’s escorting cruisers. Admiral Hipper had greater success two months later, on 12 February 1941, when she found the unescorted SLS.64 convoy of 19 ships and sank seven of them. In January 1941, the formidable battle-cruisers Scharnhorst and Gneisenau, which outgunned any Allied ship that could catch them, put to sea from Germany to raid the shipping lanes in 'Berlin'. With so many German raiders at large in the Atlantic, the British were forced to provide battleship escorts to as many convoys as possible. This twice saved convoys from slaughter by the German battle-cruiers. In February, the old battleship Ramillies deterred an attack on the HX.106 convoy, and one month later, the SL.67 convoy was saved by the presence of Malaya.

In May, the Germans mounted their most ambitious raid of all in 'Rheinübung'. The new battleship Bismarck and the heavy cruiser Prinz Eugen put to sea to attack convoys. A British force intercepted the raiders off Iceland,and in the 'Battle of the Denmark Strait', the battle-cruiser Hood was blown up and sank, but Bismarck was damaged and had to run for a French port. Bismarck nearly reached her destination, but was disabled by an air attack from the carrier Ark Royal, and then sunk by the Home Fleet on the following day. Her sinking marked the end of the warship raids. The advent of long-range search aircraft, notably the unglamorous but versatile Consolidated PBY Catalina twin-engine machine, largely neutralised surface raiders.

In February 1942, Scharnhorst, Gneisenau and Prinz Eugen moved from Brest back to Germany in 'Cerberus', the so-called 'Channel dash'. While this was an embarrassment for the British, it was the end of the German surface threat in the Atlantic. The loss of Bismarck, the destruction of the network of supply ships that supported surface raiders, the repeated damage to the three ships by air raids, the entry into the war of the USA, the start of the Arctic convoys, and the perceived invasion threat to Norway had persuaded Hitler and the naval staff to withdraw.

War had come too early for the German naval 'Plan Z' expansion project. Battleships powerful enough to destroy any convoy escort, with escorts able to annihilate the convoy, were never achieved. Although the number of ships the raiders sank was relatively small compared with the losses to U-boats, mines and aircraft, their raids severely disrupted the Allied convoy system, reduced British imports, and strained the Home Fleet.

The disastrous convoy battles of October 1940 forced a change in British tactics. The most important of these was the introduction of permanent escort groups to improve the co-ordination and effectiveness of ships and men in battle. British efforts were helped by a gradual increase in the number of escort vessels available as the old ex-US destroyers and the new British- and Canadian-built 'Flower' class corvettes were now coming into service in numbers. Many of these ships became part of the huge expansion of the Royal Canadian Navy, which grew from a handful of destroyers at the outbreak of war to take an increasing share of convoy escort duty. Others of the new ships were crewed by Free French, Free Norwegian and Free Dutch men, but these were a tiny minority of the total number, and directly under British command. By 1941 US public opinion had begun to swing against Germany, but the war was still essentially the UK and its empire against Germany.

Initially, the new escort groups consisted of two or three destroyers and six corvettes. Since two or three of the group would usually be in dock as weather and/or battle damage was repaired, the groups typically sailed with about six warships. The training of the escorts also improved as the realities of the battle became obvious. A new base was established at Tobermory in the Hebrides islands group off the west coast of Scotland, under the command of Vice Admiral G. O. Stephenson, to prepare the new escort ships and their crews for the demands of battle.

In February 1941, the Admiralty moved the headquarters of the Western Approaches Command from Plymouth to Liverpool, where much closer contact with, and control of, the Atlantic convoys was possible. Greater co-operation with supporting aircraft was also achieved. In April, the Admiralty took operational control of Coastal Command aircraft. At a tactical level, new short-wave radar sets able to detect surfaced U-boats and were suitable for both small ships and aircraft began to arrive during 1941.

The impact of these changes first began to make their presence felt in the battles during the spring of 1941. Early in March, Prien in U-47 failed to return from patrol. Two weeks later, in the battle of the HX.112 convoy, the newly formed 3^rd Escort Group of four destroyers and two corvettes held off the U-boat pack. U-100 was detected by the primitive radar on the destroyer Vanoc, rammed and sunk. Shortly afterwards U-99 was also caught and sunk, its crew being taken prisoner. Dönitz had lost his three leading aces: Prien, Kretschme and Schepke.

Dönitz now moved his wolf packs farther to the west in order to catch the convoys before the anti-submarine escort joined. This new strategy was rewarded at the beginning of April when a pack found the SC.26 convoy before its anti-submarine escort had joined: 10 ships were sunk, but another U-boat was lost.

In June 1941, the British decided to provide convoy escort for the full length of the North Atlantic crossing. To this end, the Admiralty had asked the Royal Canadian Navy, on 23 May, to assume the responsibility for protecting convoys in the western zone and to establish the base for its escort force at St John’s, Newfoundland. On 13 June, Commodore Leonard Murray of the Royal Canadian Navy took up his post as Commodore Commanding Newfoundland Escort Force, under the overall authority of the Commander-in-Chief, Western Approaches, at Liverpool. Six Canadian destroyers and 17 corvettes, reinforced by seven destroyers, three sloops and five corvettes of the Royal Navy, were assembled for duty in the Canadian force, which escorted the convoys from Canadian ports to Newfoundland and then on to a mid-ocean meeting point to the south of Iceland, where the British escort groups assumed responsibility.

By 1941, the USA was taking an increasing part in the war, despite its nominal neutrality. In April 1941 Roosevelt extended the Pan-American Security Zone to the east almost as far as Iceland. British forces had occupied Iceland when Denmark fell to the Germans in 1940, and now the USA was persuaded to provide forces to replace the British troops on the island. US warships began escorting Allied convoys in the western Atlantic as far as Iceland, and had several hostile encounters with U-boats.

In June 1941, the USA realised the tropical Atlantic had become dangerous for unescorted US as well as British ships. On May 21, Robin Moor, a US vessel carrying no military supplies, was stopped by U-69 some 865 miles (1390 km) to the west of Freetown, Sierra Leone. After its passengers and crew had been given 30 minutes to board lifeboats, U-69 torpedoed, shelled and sank the ship. The survivors then drifted without rescue or detection for as many as 18 days. When news of the sinking reached the USA, few shipping companies felt truly safe anywhere. As Time magazine noted in June 1941, 'if such sinkings continue, U.S. ships bound for other places remote from fighting fronts, will be in danger. Henceforth the U.S. would either have to recall its ships from the ocean or enforce its right to the free use of the seas.'

A Mid-Ocean Escort Force of British, Canadian and US destroyers and corvettes was organised following the USA;s entry into the war in December 1941.

At the same time, the British were working on a number of technical developments designed to offset the German submarine superiority. Though these were British inventions, the critical technologies were provided freely to the USA, where they were then renamed and manufactured. Likewise, the USA provided the British with Catalina flying boats and Consolidated Liberator four-engined heavy bombers for use on the long-range patrol task, and these made important contributions to the war effort.

Aircraft ranges were constantly improving, but the Atlantic was far too large to be covered completely by land-based types. A stop-gap measure was instituted by fitting ramps over the bows of some of the cargo ships to create what were known as catapult aircraft merchantmen (CAM ships), equipped with a single expendable Hawker Hurricane fighter. When a German bomber approached, the fighter was launched off the end of the ramp by a large rocket in order to shoot down or drive off the German aeroplane, the pilot then ditching in the water and, it was hoped, being recovered by a ship. Nine combat launches were made, resulting in the destruction of eight Axis aircraft for the loss of one Allied pilot. Although CAM ships and their Hurricane fighters did not down a great number of aircraft, such aircraft were mostly Condor machines of the type which often shadowed the convoy out of range of the convoy’s guns, reporting back the convoy’s course and position so that U-boats could then be directed on to the convoy. The CAM ships and their Hurricane fighters thus justified the cost in fewer ship losses overall.

One of the more important technical developments of the period was shipborne direction-finding radio equipment, known as HF/DF (high-frequency direction-finding, or 'Huff-Duff'), which was fitted to escorts from February 1942 and had become common items of equipment by the spring of 1943. HF/DF let an operator determine the direction of a radio signal, regardless of whether the content could be read. Since the wolf pack tactic was dependent on U-boats reporting convoy positions by radio, there was a steady stream of messages to be intercepted. An escort could then run in the direction of the signal and attack the U-boat, or at least force it to submerge and thereby lose contact, which might prevent an attack on the convoy. When two ships fitted with HF/DF accompanied a convoy, a fix on the transmitter’s position rather than merely its bearing could be determined. However, the standard approach of anti-submarine warships was immediately to run down the bearing of a detected signal, hoping to spot the U-boat on the surface and make an immediate attack. Range could be estimated by an experienced operator from the signal strength. The target was usually found visually. If the submarine was slow to dive, the guns were used; otherwise an ASDIC search was started where the swirl of water of a crash-diving boat was observed. In conditions of good visibility a boat might try and outrun an escort on the surface while out of gun range. Running down the bearing of a HF/DF signal was also used by escort carriers (particularly the US Bogue, operating in the area to the south of the Azores islands group, sending aircraft along the line of the bearing to force the submarine to submerge by strafing and then attack with depth charges or a FIDO homing torpedo.

The British also made extensive use of shore-based HF/DF stations, to keep convoys updated with positions of U-boats.

The radio technology behind direction finding was simple and well understood by each side, but the technology commonly used before the war used a manually-rotated aerial to fix the direction of the transmitter. This was delicate work, took some time to accomplish to any degree of accuracy and, as it revealed only the line along which the transmission originated, a single set could not determine if the transmission was from the true direction or its reciprocal 180° in the opposite direction. Thus two sets were required to fix the position. Believing this stillto be the case, U-boat radio operators considered themselves fairly safe if they kept messages short. The British, however, developed an oscilloscope-based indicator which instantly fixed the direction and its reciprocal the moment a radio operator touched his Morse key. This worked simply with a crossed pair of conventional and fixed directional aerials, the oscilloscope’s display showing the relative received strength from each aerial as an elongated ellipse showing the bearing relative to the ship. The innovation was a 'sense' aerial which, when switched in, suppressed the ellipse in the 'wrong' direction leaving only the correct bearing. With this there was hardly any need to triangulate: the escort could just run down the precise bearing provided, estimating range from the signal strength, and use either efficient look-outs or radar for final positioning. Many U-boat attacks were suppressed and boats sunk in this way, a good example of the great difference apparently minor aspects of technology could make to the battle.

The manner in which Dönitz conducted the U-boat campaign required relatively large volumes of radio traffic between U-boats and headquarters. This was thought to be safe as the radio messages were encrypted using the Enigma cipher machine, which the Germans considered unbreakable. In addition, the Kriegsmarine used much more secure operating procedures than the army or air force. The machine’s three rotors were chosen from a set of eight, rather than the other services' five, and these rotors were changed every other day using a system of key sheets, and the message settings were different for every message and determined from 'bigram tables' issued to operators. In 1939, it was generally believed at the British Government Code and Cypher School at Bletchley Park that the naval Enigma could not be broken. Only the head of the German Naval Section, Frank Birch, and the mathematician Alan Turing believed otherwise.

The British codebreakers needed to know the wiring of the German navy’s special Enigma rotors, and the destruction of U-33 by the British minesweeper Gleaner, which seized some of the boat’s equipment, in February 1940 provided this information. Early in 1941, the Royal Navy made a concerted effort to assist the codebreakers, and on 9 May men of the destroyer Bulldog boarded U-110 and recovered her cryptologic material, including bigram tables and current Enigma keys. The captured material allowed all U-boat traffic to be read for several weeks, until the keys ran out. The familiarity codebreakers gained with the usual content of messages helped in breaking new keys.

Throughout the summer and autumn of 1941, Enigma intercepts combined with HF/DF made it possible for the British to plot the positions of U-boat patrol lines and so route convoys around them. Merchant ship losses dropped by more than two-thirds in July 1941, and the losses remained low until November.

During May 1943, the US Navy began using a four-rotor 'bombe' machine’s used drums for the Enigma rotors at 34 times the speed of the early British 'bombe' machines, and by September 1944, the US Navy had 121 'bombes'.

This Allied advantage was offset to a degree by the growing numbers of U-boats coming into service. The 'Type VIIC' began to reach the Atlantic in large numbers during 1941, and ey the end of the war in 1945, 568 had been commissioned. Although the Allies could protect their convoys from a time late in 1941, they were not sinking many U-boats. The 'Flower' class corvette escorts could detect and defend, but they were not fast enough to attack effectively.

In October 1941, Hitler ordered Dönitz to move U-boats into the Mediterranean Sea to support German operations in that theatre. The resulting concentration near Gibraltar resulted in a series of battles around the Gibraltar and Sierra Leone convoys. In December 1941, the HG.76 convoy sailed under escort of the 36^th Escort Group of two sloops and six corvettes under the command of Captain F. J. Walker, reinforced by Audacity, the first of the new escort carriers, and three destroyers from Gibraltar. The convoy was immediately intercepted by the waiting U-boat pack, resulting in a brutal battle. Walker was a tactical innovator, his ships' crews were highly trained and the presence of an escort carrier meant U-boats were frequently sighted and forced to dive before they could get close to the convoy. Over the next five days, five U-boats were sunk, four of them by Walker’s group, despite the loss of Audacity after two days. The British also lost one destroyer, but only two merchant ships were sunk. The battle was the first clear Allied convoy victory.

Through dogged effort, the Allies slowly gained the upper hand until the end of 1941. Although Allied warships failed to sink U-boats in large numbers, most convoys evaded attack completely and although shipping losses were high, they were manageable.

The Japanese 'Ai' carrierborne air attack on the US Pacific Fleet in Pearl Harbor and the subsequent German declaration of war on the USA had an immediate effect on the campaign. Dönitz promptly planned to attack shipping off the US east coast. He had only 12 of the larger 'Type IX' boats able to reach US waters, and half of them had been diverted by Hitler to the Mediterranean. One of the remainder was under repair, leaving only five boats for 'Paukenschlag', whose success led to the German use of the phrase 'second happy time'.

Possessing no direct experience of modern naval war just off its own shores, the USA did not employ a black-out, so U-boats simply stood off shore at night and picked out ships silhouetted against city lights. Admiral Ernest J. King, the commander-in-chief of the US Fleet and who disliked the British, initially rejected Royal Navy recommendations for a coastal black-out or a convoy system. King has been criticised for this decision, but his defenders argue the US destroyer fleet was limited (partly as a result of the sale of 50 old destroyers to the UK earlier in the war), and King claimed it was far more important that destroyers protect Allied troop transports than merchant shipping. His ships were also busy convoying Lend-Lease matériel to the USSR, as well as fighting the Japanese in the Pacific. King could not require coastal black-outs as it was the US Army which had legal authority over all civil defence, and did not follow the advice of the Royal Navy and Royal Canadian Navy that even unescorted convoys would be safer than merchant vessels sailing individually. No troop transports were lost, but merchant ships sailing in US waters were left exposed and suffered accordingly. The UK eventually had to build coastal escorts and provide them to the US in a 'reverse Lend-Lease' arrangement as King himself was unable, or perhaps unwilling, to make any such provision himself.

The first U-boats reached US waters on 13 January 1942. By the time they withdrew on 6 February, they had sunk 156,939 tons of shipping without loss. The first batch of 'Type IX' boats was followed by more boasts of the class and by 'Type VII' boats supported by 'Type XIV' 'mich cow' boats to provide refuelling and resupply at sea. The U-boats sank 397 ships totalling more than 2 million tons. In 1943, the USA launched more than 11 million tons of merchant shipping, a figure which declined in the later war years as priorities shifted.

In May, and by this time the Chief of Naval Operations as well as the commander-in-chied of the UF Fleet, King finally managed to gather sufficient ships to institute a convoy system. This quickly led to the loss of seven U-boats. The USA lacked the number of warships to cover all the gaps, and the U-boats continued to operate freely during the 'Battle of the Caribbean' and throughout the Gulf of Mexico , in which they effectively closed several US ports, until July, when the British-loaned escorts began to become available. These included 24 armed anti-submarine trawlers crewed by the Royal Naval Patrol Service, many of whose men had previously been peacetime fishermen. On 3 July 1942, one of these trawlers, Le Tigre, proved her worth by picking up 31 survivors from the US merchant vessel Alexander Macomb. Soon after this, Le Tigre managed to hunt down U-215, which had torpedoed the merchant ship and was then sunk by the British destroyer Veteran. The institution of an interlocking convoy system on the US east coast and in the Caribbean Sea in the middle of 1942 resulted in an immediate drop in attacks in those areas. As a result of the increased coastal convoy escort system, the U-boats' attention was shifted back to the Atlantic convoys. For the Allies, the situation was serious but not critical throughout much of 1942.

'Paukenschlag' had one other effect. It was so successful that Dönitz’s policy of economic war was seen, even by Hitler, as the only effective use of the U-boat, and Dönitz was given complete freedom to use the boats as he saw fit. Meanwhile, Hitler sacked Raeder after the embarrassing 'Battle of the Barents Sea', in which two German heavy cruisers had been beaten off by half a dozen British destroyers, and Dönitz was eventually promoted to Grossadmiral, and all building priorities turned to U-boats.

With the USA finally arranging convoys, losses to the U-boats quickly dropped, and Dönitz realised that his U-boats would be better employed elsewhere. On 19 July, he ordered the last boats to withdraw from the USA’s east coast, and by the end of July 1942 he had shifted his attention back to the North Atlantic, where Allied aircraft could not provide cover in what was known as the 'Black Pit'. The SC.94 convoy marked the return of the U-boats to the convoys from Canada to the UK. The command centre for the U-boats operating in the west, including the Atlantic Ocea, also changed, moving to a newly constructed command bunker at the Château de Pignerolle just to the east of Angers on the Loire river in western France. The headquarters was commanded by Kapitän Hans-Rudolf Rösing.

There were enough U-boats spread across the Atlantic Ocean to allow several wolf packs to attack many different convoy routes. Often as many as 10 to 15 boats would attack in one or two waves, following convoys like SC.104 and SC .07 by day and attacking by night. Convoy losses quickly increased and in October 1942, 56 ships totalling more than 258,000 tons were sunk in the 'air gap' between Greenland and Iceland.

U-boat losses also climbed. In the first six months of 1942, 21 boats were lost, less than one for every 40 merchant ships sunk. In August and September, 60 were sunk, one for every 10 merchant ships sunk, almost as many as in the previous two years.

On 19 November, Admiral Sir Percy Noble was succeeded as commander=-in-chief of the Western Approaches Command by Admiral Sir Max Horton. Horton used the growing number of escorts becoming available to organise 'support groups' for the reinforcement of convoys that came under attack. Unlike the standard escort groups, the support groups were not directly responsible for the safety of any particular convoy. This gave them much greater tactical flexibility, allowing them to detach ships to hunt submarines spotted by reconnaissance or detected by HF/DF. Where standard escorts had to break off an engagement to remain with their convoy, the ships of support groups could keep hunting a U-boat for many hours. One tactic introduced by Walker was the 'hold-down', in which a group of ships patrolled over a submerged U-boat until its air ran out and it was forced to surface. This might take two or three days.

Between February 1942 and July 1945, about 5,000 naval officers played war games at the Western Approaches Tactical Unit. Many pf the graduates of the tactical unit believed that the battle they fought on the unit’s linoleum floor was essential to their subsequent victory at sea.

At the start of World War II, the depth charge was the only weapon available to a vessel for destroying a submerged submarine. Depth charges were dropped over the stern and thrown to the side of a warship travelling at speed. Early models of ASDIC/Sonar searched only ahead, astern and to the sides of the anti-submarine vessel that was using it: there was no downward-looking capability. So there was a time lag between the last fix obtained on the submarine and the warship reaching a point above that position. Then the depth charges had to sink to the depth at which they were set to detonate. During those two delays, a capable submarine commander could manoeuvre rapidly to a different position and avoid the attack. The depth charges then left an area of disturbed water, through which it was difficult to regain ASDIC/Sonar contact. In response to this problem, one of the solutions developed by the Royal Navy was the ahead-throwing anti-submarine weapon, of which the first was Hedgehog.

Hedgehog was a multiple spigot mortar, which fired a pattern of 24 65-lb (29-kg) contact-fused bombs ahead of the firing ship while the target was still within the ASDIC/sonar beam. The installation of this weapon on anti-submarine ships began late in 1942. The warship could approach slowly as it did not have to clear the area of exploding depth charges to avoid damage, so its position was less obvious to the submarine commander as it was making less noise. Because a Hedgehog bomb exploded only if it hit the submarine, in the event that the target was missed, there was no disturbed water to make tracking difficult, and contact had not been lost in the first place.

The Squid was an improvement over the Hedgehog, and was introduced late in 1943. A three-barrelled mortar, it projected three 440-lb (200-kg) charges ahead or abeam. The charges' firing pistols were automatically set just before launch, and more advanced installations had the Squid linked to the latest ASDIC/sonar sets so that the Squid was fired automatically.

Detection by radar-equipped aircraft could suppress U-boat activity over a wide area, but an aeroplane attack could only be successful in conditions of good visibility. U-boats were relatively safe from aircraft at night for two reasons: firstly, the radar then in use could not detect boats at less than 1 mile (1.6 km), and secondly, the deployment of flares to illuminate the scene of any attack gave a warning sufficient to the target to make evasive manoeuvres. The introduction of the Leigh Light by the British in January 1942 solved the second problem, thereby becoming a significant factor in the 'Battle for the Atlantic'. Developed by an RAF officer, Wing Commander H. de V. Leigh, it was a powerful and controllable searchlight mounted primarily to Vickers Wellington twin-engined and Liberator four-engined bombers. These aircraft first made contact with a U-boat using air-to-surface-vessel radar. Then, about 1 mile (1.6 km) from the target, the Leigh Light was switched on to effect the immediate and accurate illumination of the target and providing U-boat commanders less than 25 seconds to react before they were attacked with depth charges. The first confirmed kill using this technology was that of U-502 on 5 July 1942.

The Leigh Light enabled the British to attack boats on the surface at night, forcing German and Italian commanders to remain under the water especially when coming into port at bases in the Bay of Biscay. U-boat commanders who survived such attacks reported a particular fear of this weapon system since aircraft could not be seen at night, and the noise of an approaching aircraft was inaudible above the din of the boat’s Diesel engines. Subsequently, the common practice of surfacing at night to recharge batteries and refresh air was mostly abandoned as it was safer to perform these tasks during daylight hours when attacking aircraft could be spotted. A drop in Allied shipping losses from 600,000 to 200,000 tons per month was attributed to this device.

By August 1942, U-boats were being fitted with radar detectors to enable them to avoid sudden ambushes by radar-equipped aircraft or ships. The first such receiver, named Metox after its manufacturer in German-occupied France, was capable of picking up the metric radar bands used by the early radars. This enabled U-boats not only to avoid detection by Canadian escorts, which were equipped with obsolescent radar sets, but allowed them to track convoys in which these sets were in use. However, it also caused problems for the Germans, as it sometimes detected stray radar emissions from distant ships or planes, causing U-boats to submerge when they were not in actual danger, preventing them from recharging batteries or using their surfaced speed. Metox provided U-boat commanders with an advantage that had not been anticipated by the British. The Metox set beeped at the pulse rate of the hunting aeroplane’s radar, about once per second. When the radar operator came within 9 miles (14 km) of the U-boat, he changed the range of his radar. With the change of range, the radar doubled its pulse repetition frequency and as a result, the Metox beeping frequency also doubled, warning the commander that he had been detected.

In 1941, US intelligence informed Rear Admiral J. H. Godfrey, the director of British naval intelligence., that the British naval codes could be broken. In March 1942, the Germans broke Naval Cipher No. 3, the code for Anglo-US communication, and some four out of five Admiralty messages from March 1942 to June 1943 were read by the Germans. As a result, sinkings of Allied merchant ships increased dramatically. Fregattenkapitän Günter Hessler, Dönitz’s son-in-law and first staff officer at U-boat command, later said that 'We had reached a stage when it took one or two days to decrypt the British radio messages. On occasions only a few hours were required. We could sometimes deduce when and how they would take advantage of the gaps in our U-boat dispositions. Our function was to close those gaps just before the convoys were due.' The codebreakers of Bletchley Park assigned only two people to an evaluation of whether or not the Germans had broken the code, and after five months they finally determined that the codes had been broken. In August 1942, the Admiralty was informed but did not change the codes until June 1943. Captain R. Dreyer, the deputy staff signals officer at Western Approaches Command, said that 'Some of their most successful U-boat pack attacks on our convoys were based on information obtained by breaking our ciphers.

On 1 February 1942, the Kriegsmarine switched the U-boats to the new 'Triton' Enigma network using the new four-rotor Enigma machine. This new key could not be read by codebreakers, and the Allies no longer knew where the U-boat patrol lines were. This made it far more difficult to evade contact, and the wolf packs ravaged many convoys. This state persisted for 10 months. To obtain information on submarine movements the Allies had to make do with HF/DF fixes and decrypts of Kriegsmarine messages encoded on earlier Enigma machines. These messages included signals from coastal forces about U-boat arrivals and departures at bases in France, and the reports from the U-boat training command. From these clues, Commander C. R. N. Winn’s Admiralty Submarine Tracking Room supplied their best estimates of submarine movements, but this information was not enough. Then on 30 October, men of the British destroyer Petard salvaged Enigma material from U-559 as she foundered off Port Said. This allowed the codebreakers to break 'Triton', a feat credited to Alan Turing. By December 1942, Enigma decrypts were again disclosing U-boat patrol positions, and shipping losses declined dramatically once more.

Following the 'Chariot' raid on St Nazaire on 28 March 1942, Raeder decided the risk of further amphibious attack was high and relocated the U-boats' western command centre to the Château de Pignerolle, where a command bunker was built and from which all Enigma radio messages between the German command system and the operational U-boats in the Atlantic Ocean were transmit