Ultra intelligence in the Battle of the Atlantic.

Ultra intelligence submarine
Above Us The Waves – a classic!


Who doesn’t love a good old fashioned WW2 submarine film? From the gripping Das Boot, and the sublime Above Us the Waves , to the frankly ridiculous U571, the stories of the men who fought at the sharp end of the Battle of the Atlantic have provided filmgoers with decades of entertainment. In more recent years, films like Enigma and The Imitation Game have taken us deep into the world of the Bletchley Park codebreakers (well, cryptographers actually – there’s a difference). In popular culture, the breaking of the German Enigma encryptions, and the subsequent intelligence produced (known as Ultra) handed an Atlantic victory to the Allies on a plate. And we all know Jon Bon Jovi captured the Enigma machine, right?

Wrong. On both counts.

Control of the seas, and denial of areas of it to one’s enemies is critically dependent on reconnaissance and intelligence.[1] In order to understand the Battle of the Atlantic, and the role of Ultra intelligence[2] in informing and shaping Allied strategy and operations, the Battle needs to be viewed through an economic lens.

Historians continue to argue about the impact of Ultra and indeed the important of the Battle itself. Gardner observes that there can be few parts of the Second World War which can have been so intimately connected with economics [3]; the Battle was largely a series of campaigns to do with the relocation of substantial amounts of goods, fuel, weapons, material (and later manpower) from one side of the ocean to the other. Economic calculation was at the very heart of strategic and operational decision-making on both sides. ‘National economies were at once enablers and limiters of grand strategy; they provided, or failed to provide, the resources with which campaigns were fought.’[4]

Gardner describes the ‘asymmetric’ nature of the Battle of the Atlantic; for the Allies, the loss of their ability to use the ocean and to connect the industrial and economic power of the United States to the European theatre would have ruled out any useful form of Allied victory. The Battle ‘had to be won and it had to stay won.’[5] From the German point of view, the purpose of the anti-shipping campaign was ultimately to deal a ‘mortal strategic blow’ by preventing the build-up of Allied land forces for the European war and to deprive Britain of essential trade supplies.

Overy argues that no Allied strategy at all was possible across the Atlantic without the defeat of the submarine.[6] Milner points out that apart from a brief period in 1943, the whole object of Allied trade movements across the Atlantic was avoidance of the enemy; to do that, the Allies had to combine effective control over their merchant shipping with naval intelligence.[7] Ultimately the Battle was won because most convoys lost no ships at all.[8]

Bletchley Park first broke the German naval Enigma on 12 March 1941. By the end of that year, 25,000 Kriegsmarine messages had been read; by the end of the war, around half a million.[9] The ebb and flow of information continued for the duration of the war – periods of illumination interspersed with periods of ‘blackout’, for example the introduction of the fourth rotor on the Enigma machines in February 1942, and when the U-boats made the switch from the Hydra cipher system to Triton, which wasn’t cracked until December 1942.

A further brief ‘blackout’ in March 1943 was resolved in just 10 days; this coincided with the last month of any significant victories for the U-boats in the Atlantic campaign. But how did the availability Ultra intelligence influence this strategic and operational decision-making?

Ultra intelligence enigma machine
An Original 4-Rotor enigma machine

Since the revelation of the existence of Ultra intelligence in the early 1970s, particularly in Winterbotham’s The Ultra Secret[10] (its existence had only previously been hinted at in the official histories by Roskill and Morison), it has taken on an almost mythical status. Much of the historiography leans towards the ‘Ultra as magic ingredient’ school of thought, typified by Calvocoressi. His view that Ultra could sway battles and save lives and gave the Allies ‘an unparalleled insight into enemy dispositions, capabilities and intentions’ has been mirrored by numerous historians since.[11] Calvocoressi even goes as far as to claim that without Ultra, the Battle of the Atlantic would’ve been won by Donitz,[12] but this is both an exaggeration and an over-simplification. More recent commentators have taken a more critical view:

 ‘…many of the claims made for the significance of Ultra over the last quarter of a century are overdrawn at best and specious at worst.’[13]

The British intelligence official history was not published until 1988. This takes a more measured approach, but by then the ‘Ultra Myth’ was already well established in both popular and academic consciousness. The achievements of the cryptographers at Bletchley Park have now become an essential part of the British narrative of the war.[14] More recently, Ferris has argued that Ultra intelligence did little to cause Axis defeat, but much to shape Allied victory.[15]

This article will consider the role of Ultra intelligence in shaping the strategy and operations of the Western Allies. The role of Ultra in convoy warfare will be discussed, as well as its use in specific operations between 1941 and 1943, and the part it played in the eventual withdrawal of the U-Boat fleet from the North Atlantic. And neither Jon Bon Jovi nor Benedict Cumberbatch get a mention…

Turing, ultra intelligence, Battle of the Atlantic
Alan Turing

1941: Ultra and the Convoy System

The fall of France gave the Germans access to the Atlantic bases in the Bay of Biscay; from July 1940 until the Spring of 1941, the U-boats ran riot amongst Allied shipping, picking off independent ships and the poorly protected early convoys. The British had lost a number of surface ships at Dunkirk, and the threat of invasion meant fewer ships were available for escort duty. 1941 saw the tripling of the U-boat fleet but as the year progressed, the British made increasing use of the newly obtained Ultra intelligence to plan and re-route convoys.

According to Calvocoressi, Ultra ‘enabled the Admiralty to play hide and seek in the Atlantic with its eyes open.’ However, it could do so only if it were received ‘in time.’ Intelligence decrypted 2 or 3 weeks of receipt was of little use in making operational, or indeed strategic decisions. At this time, the essence of Allied convoy operations in the Atlantic was the avoidance of the enemy; engagements were to be avoided at all costs.

When available in time (a period of 48 hours is now widely considered the maximum window between sending and decryption),Ultra did allow a more reliable evasive routing of convoys. Whether Ultra picked up the movements of an individual U-boat, or a pack operating under Rudeltaktik, toward a convoy, the Naval Control Service (a sort of ‘air traffic control’ for shipping) could issue orders deflecting the route of the convoy north or south, to ‘swerve’ the waiting Germans. Good naval intelligence allowed enemy U-boat patrols to be tracked, as against Allied shipping movements, and naval forces to be deployed to intercept them or protect shipping.

Ultra intelligence, Atlantic convoy
An Atlantic convoy viewed from a Short Sunderland, circa 1943.

Ultra was a major factor in the early phases of the Atlantic campaign, but it was not the only one. As Gardner points out, it was perfectly possible for the Germans to fail to find convoys to attack even when the Allies did not have access to Ultra, and they had their own intelligence, access to the British Naval cyphers, and reconnaissance to assist.[16]

It should also be borne in mind that the U-boats were having to operate further and further out into the Atlantic, at great distances from their bases. This required constant return trips for re-supply, refuelling (until the introduction of the milch kuhe refuelling vessels), and re-armament, and many of the U-boat fleet were diverted away from the Atlantic to operations in the Mediterranean. Add to this the increasing involvement of the US in escort activities, and a picture emerges of initial German success in the Atlantic gradually being eroded.[17]

However, as the campaign progressed, Ultra intelligence paid what Overy describes as “rich dividends”; for example, between May 1942 and May 1943 105 out of 174 convoys sailed without any U-Boat interference; out of the 69 where contact was made, 23 were not attacked and 30 suffered only minor losses; only the remaining 10% were severely depleted.[18] The majority of these losses occurred during the “blackout” period when Ultra failed.

Ultra and Operations against the German Surface Fleet

Much of the attention in studies of the Battle of the Atlantic has been devoted to the U-boat war against merchant tonnage, however in the early part of the war the German surface fleet was considered to pose a significant threat to shipping. Huge resources and numbers of ships and aircraft were dedicated to the search for, and attempted destruction of, the German surface fleet, resources which could (and perhaps should), have been better employed elsewhere.

Gardner describes the Allied response to the perceived threat posed by enemy surface vessels as an ‘over-reaction’ which led to the mobilisation of all available battleships, aircraft carriers and cruisers, as well as many other land-based units and aircraft, which occasionally had to be re-deployed from other theatres, in particular the Mediterranean.[19]

However, the increasing availability of Ultra intelligence meant that resources could be better managed, rather than simply adopting a ‘kitchen sink’ approach. The tracking and sinking of the Bismarck (26/27 May 1941) illustrates a very early operational use of timely Ultra, working in conjunction with other types of intelligence. Visual intelligence by Coastal Command patrols, photographic reconnaissance, an increase in low-grade radio traffic picked up by the Y-stations, and ultimately signal direction finding, gave away the Bismarck’s position.

Ultra contributed by disclosing Luftwaffe preparations to provide air cover. The sighting of the Bismarck by a Coastal Command Catalina was effected by a combination of three types of intelligence: signal direction finding, analysis of radio and wireless traffic, and Ultra, although Lewin plays down Ultra’s role and points out that not one signal to or from Bismarck was ever deciphered in time.[20]

Where Ultra undoubtedly did assist was in the tracking down and sinking of the tankers and supply ships which had been tasked with supporting Bismarck and Prinz Eugen, on 23 June 1941.[21] The destruction of the Kriegsmarine’s flagship was not by any means a victory for Ultra alone, which was in reality a bit-part player in the incident, but perfectly illustrates the symbiotic relationship between Ultra and other types of intelligence (and their potential), when correctly interpreted and applied decisively at operational level.

As time went on, the intelligence handlers became more adept, and a broader and more detailed picture of enemy strategy and operations could be gathered, allowing for much more focused allocation of Allied resources against enemy surface (for example, in relation to the sinking of the Scharnhorst on 25/26 December 1943) and U-boat fleets.

Ultra intelligence, The Bismarck
The Bismarck in 1940

1942: Blackout, Torch and Triton

According to Mawsdley, the war in the Atlantic in 1942 was in many ways different from that of the previous year; the Royal Navy had a powerful new ally across the Atlantic. The nature of the Atlantic battle changed hugely, particularly with the attacks on unprotected shipping off the US eastern seaboard and the initial unwillingness of the Americans to adopt the convoy system, for which they paid a heavy price. In the first 6 months of 1942, 2.34 million tons of shipping were lost on the Eastern seaboard, and in the Gulf and Caribbean.[22]

However, the nature of both organising shipping and the battles between escorts and attackers went on, with only incremental changes.[23] In 1941, the Allies had lost 4.3 million tons of shipping; in 1942 this doubled to almost 7.8 million tons, and the U-boat fleet had doubled with around 100 Frontboote being kept out at sea at any one time.[24]  300 U-boats were available at the beginning of 1942, and almost 400 by the end of the year.[25] Although Donitz had set a target of 800,000 tons of shipping to be sunk per month, this target was only exceeded 3 times, latterly in November 1942, just a month before Triton was finally cracked.

The eventual adoption of the convoy system for Atlantic crossings by the Americans, together with the adoption of a coastal blackout, radio silence, and air patrols, meant that the U-boats were gradually forced away from US coasts and by late 1942, back into the mid-Atlantic air-gap, where there were still ‘easy pickings’ available, resulting in Allied Atlantic shipping losses of 5.4 million tons for the year.[26]

For the Allies, the sinking of merchant ships was inextricably linked to the rate at which they could be built; after the Americans entered the war, shipbuilding, particularly in the US expanded exponentially. Ships were being built at a much faster rate than the U-boats could sink them,[27] with twenty-one million tons completed in the American shipyards between 1942 and 1945.

In terms of Allied strategy, the Battle of the Atlantic in 1942 went beyond the concepts of the tonnage war and ferrying supplies to Britain and Russia; it was now about the use of the ocean for decisive military strategies.[28] In Spring 1942 the US began the movement across the Atlantic of ground and air forces to Britain, the former to begin the build-up of personnel for a possible European invasion and later for the invasion of North Africa (Operation TORCH), the latter to take part in the Strategic Bombing Campaign against Germany. In order to find the shipping to support TORCH, the British had to shut down convoys in the South and East Atlantic, which meant trade was largely restricted to the North Atlantic route; this in turn meant U-boats poured into the area.

Ferris argues that ideology, perceptions, military rationality, and open information drove decisions about strategy more than did secret intelligence; in operations, intelligence served as a ‘force multiplier.’[29] For much of 1942, the German enigma transmissions could not be read following the switch to the Triton cypher, although many other sources of intelligence (including the general purpose German naval Hydra cypher continued to be read) were available to the Allies.[30] Ultra had contributed to the reduction of shipping losses in 1941; combined with the massive increase in shipping resources being made available to the Allies, this at least allowed the Allies to begin planning their strategy for the next phases of the wider war, and the physical transfer of men and materiel across the Atlantic to put these plans into practice.

1943: Seeing the bigger picture

Ultimately it was the preparations for the inevitable invasion of Europe, decided upon at Casablanca in January 1943, rather than any threat against British trade supplies in late 1942, which forced the hands of the Allies into resolving the Atlantic issue.[31] The result was the closing of the Atlantic air gap, with the provision of more VLR aircraft, more radar-equipped escorts and small aircraft carriers. Ultra intelligence also revealed that the British Convoy code system had been read by the German B-Dienst service for years; the system was changed on 15 December 1942, thereby depriving the Germans of what had been the most useful method for positioning their U-boats for some time.[32]

The breaking of Triton in December 1942 eventually assisted the Allies to move from a strategy of defence and avoidance to open aggression and offense. Allied policy was no longer simply one of avoiding the U-boats, but rather one of drawing them out and destroying them in offensive action, and there is no doubt that Ultra played a role in helping the Allies to decide where best to focus their newly available air and naval resources. Lewin describes how Ultra gave an insight into the whole ‘life-cycle’ of the U-boat, from training to destruction.[33]

In the first two months of 1943, U-boat operations in the North Atlantic were curtailed both by severe weather, and by the successful re-routing of convoys because of Ultra. However, Ultra failed completely, but briefly, in March 1943; during this time 100% of Atlantic convoys were intercepted, 54% attacked and 22% of all shipping in those convoys was sunk,[34] with the infamous attacks on HX.229 and SC.122 resulting in the loss of twenty-two merchant vessels on 16-20 March 1943. Gardner argues that it was not the disasters of March 1943 nor the driving off of the U-boats from the North Atlantic in May 1943 which marked the ‘tipping point’ of the Atlantic battles, but rather that it was the winter of 1942/3 when the German strategic grip began to slip.[35]

Ultra was restored in late March, allowing for effective re-routing of the majority of convoys, but also allowing for certain convoys to be heavily reinforced. The Allies didn’t need to track the progress of every submarine over every mile, just the general trend of where it was aiming for and when it might get there; this allowed them to allocate their efforts into the right area and put appropriate measures in place.[36] By the end of May 1943, amid crippling U-boat losses, the Germans were forced to alter their strategy and the U-boats were withdrawn from the mid-Atlantic.

A typical ‘milch kuh’, type XIV U-boat supply tanker

Between May and September 1943, the Germans lost eight of their twelve tankers. Ultra played a key role in this operation, despite British fears that the Americans were too ‘gung-ho’ in their use of the intelligence and threatened to alert the Germans that their cyphers had been broken. In fact, it has been argued that the systematic elimination of U-tankers was a policy largely made possible by Ultra.[39]

In addition to attacks on German U-tankers, the Allies were able to focus their attention on German activities in the Bay of Biscay from mid-1943, targeting U-boats as they moved in and out of the French Atlantic ports. Ultra decryption of routine, mundane signals could often give an indication of arrivals and departures, thereby helping the Allies to build up a general picture of trends and operations in the Bay.

However, Ultra also had another use –monitoring the various shifts in perception and policy implemented by the BdU, and even the morale of the U-boat crews. Gardner notes that the Allied success rate in the Bay was greatly facilitated by two linked factors. Firstly, the German’s own interpretation of their defeat in the Mid-Atlantic in May and their planned strategy for dealing with this; secondly, the Allies’ ability to devote fresh resources (naval and air) to the Bay area.[40]

At operational level, Ultra also allowed the Allies to deliberately target the U-boat fuel tankers; these had of course been sunk occasionally prior to May 1943, but any such sinkings tended to be incidental rather than pre-planned. Gardner argues that a deliberate policy of seeking out the ‘milch kuhe’ prior to then could only have been implemented at the expense of convoy protection. For much of the July 1942-May 1943 period Ultra was probably insufficiently timely to have allowed for the systematic targeting of U-tankers.[37]  However, the changes in German strategy from the end of May 1943 demanded increased reliance on refuelling; the withdrawal of the U-boats from mid-Atlantic convoy warfare freed up Allied resources which could then be re-focused on hunting down the U-tankers, guided by Ultra intelligence. Lewin describes the attacks on the milch kuhe as illustrating exactly ‘the practical value of the Ultra system when it was functioning at its best.’[38]

‘At low tactical levels Ultra was of little use, rarely being timely and accurate enough to influence matters much…matters were different at the operational level where much, if not all, of the information on which tactics and the allocation of forces was based came from Ultra.’[41]

Donitz’s withdrawal from the mid-Atlantic from 24 May 1943 has been described as the ‘hinge’ in the Battle of the Atlantic. [42] That Allied forces in the Atlantic were very quickly cognisant of the change, as a result of Ultra, meant that operational level shifts were possible, for example the reinforcement of USA-Mediterranean convoys and the resultant preparations for the Allied invasion of Sicily in 1943, and the re-allocation of ships and aircraft to the Biscay area. German submarines became prey rather than hunters. Gardner argues that such a series of operational-level changes cumulatively constituted a ‘strategic gear-shift,’ and that the main enabling factor of that shift was Ultra.[43]

Extract from logbook of Sgt Jack Clyde, RAF, on detachment to Coastal Command, searching for U-boats in the Bay of Biscay. March 1943.

Other Factors

Ultra should not be viewed as a ‘stand alone.’ It was only one of many advantages and tools and structures (not least the establishment of the Naval Control Service and the Western Approaches Command system) which the Allies gradually created for themselves. The increased availability of long-range aircraft in 1943 and the resultant closure of the ‘air gap,’ the roll out of high frequency direction-finding, new types of radar and innovative navigational, search and weapons technologies on ships and aircraft, such as the Leigh light, better trained and more experienced escort crews, the construction of vast numbers of new ships, and the changing of the British Naval ciphers at the end of 1942, and the waning effectiveness of German intelligence all played varying parts in helping the Allies to shape both their long term strategy and short-term operations.

The willingness of the Allies to share these resources and to co-operate (eventually) to win the Atlantic campaign should not be under-estimated. The availability of Ultra aided the Allies in bringing all these advantages and tools together and allowed them to efficiently focus these modern technologies and resources where they were most needed.


According to Syrett, the Allies were ultimately victorious in the Atlantic because, unlike Germany, they were not only capable of deploying new technologies but were also able to develop and adopt new methods of anti-submarine warfare with which to defeat the German attack on convoys, and finally, unlike their adversaries, were able to learn from their mistakes.[44] Milner argues that the Allied defence of shipping (in which Ultra played a part) was so overpowering that at no point between the end of May 1943 and May 1945 were Allied plans or operations threatened by the German campaign in the Atlantic.[45] By late 1943, he argues that the Allies had more shipping than they knew what to do with, as a result of the defeat of the U-boats, American productions and the opening up of the Mediterranean.[46]

Mawdsley argues that the Battle of the Atlantic was never such a ‘near run thing’ as it is often depicted, and that in fact Britain never actually experienced a crisis of supply; nonetheless, gaining and maintaining control of the Atlantic required massive and decisive effort.[47] FH Hinsley, chief editor of the official history of British Intelligence, claimed that Ultra saved the Allies 3-4 years of war and vast expenditure in lives and resources. This view is dismissed by Ferris on the basis that Hinsley’s case over-estimates Ultra’s contribution to the Battle, and of the battle to the war; he argues that the intelligence victory probably shaved months not years off the war, though he concedes that it probably did save the lives of tens of thousands of Allied personnel.[48]

Ultra was undoubtedly of immense value, but what Milner describes as ‘the humdrum of more conventional forms of operational analysis,’ such as direction finding and radio traffic analysis, played fundamental roles. One must be careful not to overplay Ultra’s part.

Where Ultra shone was in allowing a strategy of avoidance to be effective, permitting the safe routing of trade away from known enemy concentrations; it went hand in hand with the Naval Control of Shipping.[49] Beyond that, it could be argued that Ultra made only a limited contribution to Allied grand strategy –for example, the preparations for and delivery of TORCH took place regardless of the Ultra blackout which persisted for most of 1942. However, this argument fails to acknowledge the concept of ‘the strategic shift’ enabled by Ultra in the Spring of 1943 onwards. Naval power, underpinned by Ultra, gave the Allies what Overy refers to as ‘the luxury of picking easier fields of combat, against the weak links in the enemy’s armour.’[50]

Ferris postulates that the weaknesses in the German enigma system had little to do with the reasons for Germany’s defeat, though much to do with the pace of that defeat.[51] Ultra amplified the influence of the British over the formulation of Anglo-American strategy in Europe; it ‘provided mastery in the war of knowledge, which significantly multiplied Allied power and command.’[52]

To put it another way, Ultra was not the decisive factor in the Allied victory in the Atlantic; without it, there is every likelihood that the Allies would still have won, but that that victory would have been delayed, and immense resources expended, which were ultimately able to be deployed elsewhere. Ultra was an enabler – it gave the Allies choices, and a gradual and increasing advantage over German naval forces. Perhaps the true influence of Ultra is best reflected in Calvocoressi’s assessment that good intelligence conditions the two major elements in war-making:

the choice between strategies and the choice between ways of implementing the strategy you’ve chosen.’[53]

You can read more about RAF crewman Sgt Jack Clyde ( log book featured above) and his crew, in my book Above Us the Stars: 10 Squadron Bomber Command – The Wireless Operator’s Story

Reaping the Whirlwind Above us The Stars

If you’re interested in British strategy in the Battle of the Atlantic, a visit to the Western Approaches Museum in Liverpool is highly recommended.

On Second World War history generally, you might enjoy www.justcuriousjane.com/bring-up-the-piat/ and www.justcuriousjane.com/lack-of-moral-fibre

Further reading:


Peter Calvocoressi, Top Secret Ultra, (Ballentine, New York), 1980.

John Ferris, Behind the Enigma – The Authorised History of GCHQ, (Bloomsbury, London) 2020, Ch.6

WJR Gardner, Decoding History: The Battle of the Atlantic and Ultra, (MacMillan Press, London), 1999.

Ronald Lewin, Ultra Goes to War – The Secret Story, (Penguin, London), 2001.

Evan Mawdsley, The War for the Seas: A Maritime History of World War II (Yale University Press, New Haven and London), 2020, Ch. 5, 12, and 15.

Mark Milner, Battle of the Atlantic, (The History Press, Stroud), 2003.

Richard Overy, Why the Allies Won, (Jonathan Cape, London, 1995) (Pimlico, London), 2006, Ch. 2.

RA Ratcliff, Delusions of Intelligence: Enigma, Ultra and the End of Secure Ciphers, (Cambridge University Press, New York), 2006.


John Buckley, ‘Coastal Command in the Second World War’, Air Power Review, Vol: 21, No: 1, Spring 2018, pp.7-30.

WJR Gardner, ‘Prelude to Victory: The Battle of the Atlantic 1942-1943’, The Mariner’s Mirror, 1993, 79:3, pp.305-317

 WJR Gardner, ‘From Hunter to Hunted: The U-boat war in the Atlantic’, 1939-1943 (Review), The Mariner’s Mirror, 2020, 106:4, pp. 496-498.

Eric Grove, ‘The Battle of the Atlantic: A legend deconstructed’, The Mariner’s Mirror, 2019, 105:3, pp. 336-339.

Michele Magnozzi, ‘One Torpedo, One Ship: An appraisal of Otto Kretschmer’s U-boat tactics, 1939-1941’, The Mariner’s Mirror, 2021, 107:2, 202-215.

Marc Milner, ‘Naval Control of Shipping and the Atlantic War 1939-45’, The Mariner’s Mirror, 1997, 83:2, pp.169-184.

Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, Global War Studies, 14 (1), 2017, pp.45-60.

Duncan Redford, ‘The March 1943 Crisis in the Battle of the Atlantic: Myth and Reality’ History, vol. 92, no. 1 (305), Wiley, 2007, pp. 64–83

Duncan Redford, ‘Inter- and Intra-Service Rivalries in the Battle of the Atlantic’, Journal of Strategic Studies, 32:6, pp. 899-928

David Syrett, ‘The Battle for Convoy OG69, 20-29 July 1941’, The Mariner’s Mirror, 2003, 89:1, pp. 71-81.


[1] WJR Gardner, Decoding History: The Battle of the Atlantic and Ultra, (MacMillan Press, London), 1999, p.11.

[2] For the purposes of this essay, ‘Ultra intelligence’ refers to decrypted enemy naval intelligence.

[3] WJR Gardner, Decoding History, p.34.

[4] WJR Gardner, Decoding History, p.35.

[5] WJR Gardner, Decoding History, p.4.

[6] Richard Overy, Why the Allies Won, (Jonathan Cape, London, 1995) (Pimlico, London), 2006, p.54.

[7] Marc Milner, ‘Naval Control of Shipping and the Atlantic War 1939-45’, The Mariner’s Mirror, 1997, 83:2, p. 169.

[8] Eric Grove, ‘The Battle of the Atlantic: A legend deconstructed,’ The Mariner’s Mirror, 2019, 105:3, p.336.

[9] Peter Calvocoressi, Top Secret Ultra, (Ballentine, New York), 1980, p. 97.

[10] Frederick Winterbotham. The Ultra Secret. (Weidenfeld and Nicolson, London), 1974.

[11] Peter Calvocoressi, Top Secret Ultra, p.2.

[12] Peter Calvocoressi, Top Secret Ultra, p. 126.

[13] WJR Gardner, Decoding History, p.218.

[14] Evan Mawdsley, The War for the Seas: A Maritime History of World War II (Yale University Press, New Haven and London), 2020, p.97.

[15] John Ferris, Behind the Enigma – The Authorised History of GCHQ, (Bloomsbury, London) 2020, p.223.

[16] WJR Gardner, Decoding History, p. 177.

[17] WJR Gardner, Decoding History, p. 17.

[18] Richard Overy, Why the Allies Won, p.59.

[19] WJR Gardner, Decoding History, p. 8.

[20] Ronald Lewin, Ultra Goes to War – The Secret Story, (Penguin, London), 2001, p. 201.

[21] Ronald Lewin, Ultra Goes to War, p. 207.

[22] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, Global War Studies, 14 (1), 2017, p. 53.

[23] Evan Mawdsley, The War for the Seas, p. 253.

[24] Peter Calvocoressi, Top Secret Ultra, p. 104.

[25] Richard Overy, Why the Allies Won, p.55.

[26] Richard Overy, Why the Allies Won, p.58.

[27] Duncan Redford, ‘The March 1943 Crisis in the Battle of the Atlantic: Myth and Reality’, History, vol. 92, no. 1 (305), Wiley, 2007, p.67.

[28] Evan Mawdsley, The War for the Seas, p. 263.

[29] John Ferris, Behind the Enigma, p.223.

[30] Ronald Lewin, Ultra Goes to War, p.210.

[31] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, p. 55.

[32] WJR Gardner, Decoding History, p. 311.

[33] Ronald Lewin, Ultra Goes to War, p.207.

[34] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, p. 56.

35 WJR Gardner, ‘Prelude to Victory: The Battle of the Atlantic 1942-1943’, The Mariner’s Mirror, 1993, 79:3, p.312.

[36] WJR Gardner, Decoding History, p.205.

[37] WJR Gardner, Decoding History, p.206.

[38] Ronald Lewin, Ultra Goes to War, p. 207.

[39] WJR Gardner, Decoding History, p.101.

[40] WJR Gardner, Decoding History, p.208.

[41] WJR Gardner, Decoding History, p.208.

[42] WJR Gardner, Decoding History, p.209.

[43] WJR Gardner, Decoding History, p.209

[44] David Syrett, ‘The Battle for Convoy OG69, 20-29 July 1941’, The Mariner’s Mirror, 2003, 89:1, p.79.

[45] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, p. 58.

[46] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, p. 58.

[47] Evan Mawdsley, The War for the Seas, p. 476.

[48] John Ferris, Behind the Enigma, p. 265

[49] Marc Milner, ‘The Atlantic War 1939-1945: The Case for a New Paradigm’, p. 49.

[50] Richard Overy, Why the Allies Won, (Jonathan Cape, London, 1995), (Pimlico, London), 2006, p. 54.

[51] John Ferris, Behind the Enigma, p. 264.

[52] John Ferris, Behind the Enigma, p. 265.

[53] Peter Calvocoressi, Top Secret Ultra, p.127.

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