May 14, 2013

Finest Hour 149, Winter 2010-11

Page 20

Churchill and Intelligence – Golden Eggs: The Secret War, 1940-1945 / Part I: Britain and America

Churchill used secret intelligence on a global scale, freely shared it with the Americans, and made it count in the Battle of the Atlantic. The Cabinet’s unanimous decision to aid Greece in 1941 would not have been made were it not for the Enigma decrypts.

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By Martin Gilbert

The Rt. Hon. Sir Martin Gilbert CBE was named the official biographer of Sir Winston Churchill following the death of Randolph Churchill in 1968, and has now published almost as many words on his subject as Churchill himself. “Official” is something of a misnomer, since he was never told what to write or what position to take on any aspect of the story. Winston S. Churchill, with eight narrative volumes and sixteen companion or document volumes, and more to come, is the longest biography ever published. It is now being reprinted in full by Hillsdale College Press, which offers all volumes so far published at affordable prices. See: http://www.hillsdale.edu/news/free-domlibrary/churchill.asp. Sir Martin is an honorary member of The Churchill Centre and has been a contributor to Finest Hour for nearly thirty years. For further information see http://www.martingilbert.com.

Churchill became Prime Minister on 10 May 1940. Twelve days later, on 22 May, the codebreakers at Bletchley Park broke the Enigma key most frequently used by the German Air Force. This was the hourly two-way top-secret radio traffic between the combined German Army, Navy and Air Force headquarters at Zossen and the commanders-in-chief on the battlefronts.

Included in the newly broken key were the top-secret messages of German Air Force liaison officers with the German Army. The daily instructions of these liaison officers included targets, supply and, crucially, details of shortages such as aviation fuel.

In the desperate days of late May and early June 1940—when the British Expeditionary Force was being evacuated from Dunkirk—daily decrypts indicated the position and intentions of the German field formations as they turned towards the sea. Even with delays in decrypting individual Enigma messages taking, at that time, up to six days, this was an indispensable benefit in guiding the continuation of the evacuation to the last possible moment.

As the German air Blitz on Britain intensified in August and September 1940, fears of invasion mounted. On 11 September Churchill received details of an Enigma decrypt that suggested that invasion plans were being made. British Military Intelligence commented on this decrypt:

“Although there are a number of possible reasons for this order, it cannot be overlooked that it may be in connection with the movement of troops and armament for invasion purposes.”¹

Throughout the autumn and winter of 1940, the search for indications of a German invasion remained the top priority of the Bletchley eavesdroppers. On 10 October 1940, Churchill was shown summaries of decrypts of German Air Force top-secret signals that had revealed, among other German instructions, the appointment in the first week of October of German Air Force officers to the embarkation staffs at Antwerp, Ostend, Dunkirk and Calais, where air reconnaissance—another indispensable arm of Intelligence—revealed the presence of what could would well have been invasion barges.

Top-secret instructions had also been decrypted at Bletchley with regard to a German air formation headquarters, which was known to be in charge of German Air Force equipment in Belgium and Northern France, “settling the details of loading of units and equipment into ships.” On 24 September 1940 the German Third Air Fleet received orders, also decrypted at Bletchley, concerning the supply of air/sea rescue vessels by seaplane bases off Norway and along the North Sea and Channel coasts “in connection,” as the message sent to Churchill explained, “with the Seelöwe (Sea Lion) operation,” presumed to be the invasion of Britain.

On 9 October 1940 a further decrypt revealed that on the previous day the headquarters of the Second German Air Fleet “asked for provision of two tankers each filled with approximately 250,000 gallons of aviation fuel to be held in readiness for S+3 day (presumably the third day of invasion operations against UK) at Rotterdam and Antwerp.”²

It was not until 12 January 1941 that Churchill received the details of an Enigma decrypt that German Air Force wireless stations on the circuit of the air formation headquarters responsible for German Air Force equipment in Belgium and Northern France—equipment that was known to have been on standby for invasion duties—was “no longer to be manned as from January 10.”³

The danger of invasion was over. Other decrypts were now making it clear that the new German focus of military and air preparations was against its ally and partner of the previous sixteen months, the Soviet Union.⁴

Churchill’s Vigilance

As Prime Minister and Minister of Defence, Churchill was intensely concerned with maintaining the secrecy of all aspects of war policy and planning. In no area was secrecy more important to him than with regard to Enigma.

On 16 October 1940 he wrote to General Ismay, head of his Defence Office: “I am astounded at the vast congregation who are invited to study these matters. The Air Ministry is the worst offender and I have marked a number who should be struck off at once, unless after careful consideration in each individual case it is found to be indispensable that they should be informed. I have added the First Lord, who of course must know everything known to his subordinates, and also the Secretary of State for War.”

Churchill continued: “A machinery should be constructed which makes other parties acquainted with such information as is necessary to them for the discharge of their particular duties. I await your proposals. I should also add Commander-in-Chief Fighter and Commander-in-Chief Bomber Command, it being clearly understood that they shall not impart them to any person working under them or allow the boxes to be opened by anyone save themselves.”⁵

Within three weeks of Churchill’s minute to Ismay, the number of recipients of Enigma-based material had been fixed at thirty-one.

Churchill’s vigilance was continual. In September 1941, on reading the wide circulation given to a 7 a.m. summary of a series of decrypts giving the movement of German fuel ships in the Mediterranean between Naples and the North African port of Bardia, he wrote to Brigadier Stewart Menzies, Chief of the Secret Intelligence Service (MI6), and to the Army and Navy Chiefs of Staff: “Surely this is a dangerously large circulation. Why sh[oul]d anyone be told but the 3 C-in-Cs. They can give orders without giving reasons. Why should such messages go to subsidiary HQs in the Western Desert.”⁶

When Churchill travelled outside London and overseas, the summaries and assessments of Enigma decrypts were sent on to him by courier or top-secret radio signal. When he was in Britain, translated summaries of the decrypts, and the Bletchley assessments, were sent him in locked buff-coloured boxes to which he alone had the key. None of his Private Office knew what the contents were. As his Junior Private Secretary, John Colville, noted in his diary at Chequers in May 1941: “The PM, tempted by the warmth, sat in the garden working and glancing at me with suspicion from time to time in the (unwarranted) belief that I was trying to read the contents of his special buff boxes.”⁷

Churchill made his first visit to Bletchley on 6 September 1941. His Principal Private Secretary, John Martin, who accompanied him in the car on their way to Oxfordshire for the weekend, did not enter the building, and had no idea what went on there.⁸

Following his visit to Bletchley, Churchill received a letter, dated 21 October 1941, from four Bletchley cryptographers, Gordon Welchman, Stuart Milner-Barry, Alan Turing and Hugh O’D. Alexander. In their letter, they urged Churchill to authorize greater funding for the work they were doing. Manual decoding was extremely time-consuming. Turing believed that a machine he had devised—the “bombe,” then in its early days—could speed up the task considerably, but that more funding and more staff were needed. Milner-Barry later explained: “The cryptographers were hanging on to a number of keys by their coattails and if we had lost any or all of them there was no guarantee (given the importance of continuity in breaking) that we should ever have found ourselves in business again.”⁹

In view of the exceptional secrecy, Milner-Barry took the letter by hand to 10 Downing Street. He later reflected: “The thought of going straight from the bottom to the top would have filled my later self with horror and incredulity.” On receipt of the letter, Churchill wrote to the head of his Defence Office, General Ismay (his letter marked “Action This Day”): “Make sure they have all they want on extreme priority and report to me that this has been done.”¹⁰

“Almost from that day,” Milner-Barry recalled, “the rough ways began to be made smooth. The flow of bombes was speeded up, the staff bottlenecks relieved, and we were able to devote ourselves uninterruptedly to the business in hand.” Brigadier Menzies—who rebuked Gordon Welchman when they met for having “wasted fifteen minutes” of the Prime Minister’s time—reported to Churchill on November 18th that every possible measure was being taken. Bletchley’s needs were met.

Churchill, fully aware of the crucial role of Bletchley Park in averting defeat—and in due course, if all went well on the battlefield, to secure victory—had ensured that funds would be made available to improve the bombe, which was decisively to accelerate the decrypting of Enigma messages.

In the second week of March 1943, Enigma decrypts of German dispositions in the Mediterranean disclosed that four merchant vessels and a tanker, whose cargoes were described by Field Marshal Kesselring as “decisive for the future conduct of operations” in North Africa, would sail for Tunisia on March 12th and 13th, in two convoys.

Alerted by this decrypt, British air and naval forces sank the tanker and two of the merchant ships.

Unfortunately, before despatching the intercepting force, the British planners of the operation failed to provide sufficient alternative sightings, so as to protect the Enigma source. An Enigma decrypt on March 14th made it clear that the suspicions of the German Air Force had been aroused, and that a breach of security was being blamed for the loss of the vital cargoes. Churchill, reading this decrypt, minuted at once that the Enigma should be withheld unless it was “used only on great occasions or when thoroughly camouflaged.”¹¹

Fortunately for Britain, the Germans did not suspect that their Enigma secret was the cause of this apparent breach of their security. Nor did the Germans manage to break into Britain’s own Signals Intelligence system. Had they done so, they would have learned at once that Enigma had been compromised.

What to Tell the Americans?

Following the visit to Britain of President Roosevelt’s emissary Harry Hopkins, in January 1941, Churchill agreed that the United States could share information concerning Enigma, and could do so without delay. In February 1941 the Currier-Sinkov mission from the United States brought a Japanese Foreign Office “Purple” cypher machine and other codebreaking items to Bletchley, where Colonel Tiltman’s solutions of Japanese army code systems, which he explained to the American cryptographers during their visit, represented the first solutions of Japanese army material that United States cryptanalysts had seen.¹²

In his own reading of Enigma, Churchill was always on the lookout for items that he felt should be sent to Roosevelt. Especially with Enigma decrypts and interpretations that related to the Far East and the Pacific, he would note for Brigadier Menzies: “Make sure the President knows this” or “make sure the President sees this.”¹³

In April 1942, four months after the Japanese attack on Pearl Harbor, Churchill authorized the visit by Colonel Tiltman to OP-20-G, the United States Navy’s cryptanalytic office in Washington DC.¹⁴ During Tiltman’s visit it became clear that the United States Navy wanted to attack the German naval “Shark” key, against which Bletchley had made almost no progress since the introduction of the four-rotor Enigma (M4) on 1 February 1942: this Shark key provided the German Navy with all top-secret communications with its submarines.

On 8 February 1942, Churchill wrote to Menzies: “Do the Americans know anything about our machine? Let me know by tomorrow afternoon.” Colonel Menzies replied on the following day: “The American Naval Authorities have been given several of our Cypher Machines.”¹⁵

In May 1942, Churchill approved Menzies’ agreement to help OP-20-G to work on the “Shark” key and other German Enigma circuits. OP-20-G eventually built more than a hundred four-rotor bombes that proved invaluable in solving—between June 1943 and April 1945—not only 2940 German Naval Enigma keys, but also 1600 German Army and Air Force Enigma keys, all of which were then read at Bletchley without interruption.¹⁶

In June 1942, General Dwight D. Eisenhower arrived in Britain as Commander-in-Chief of the United States Forces then being assembled as a prelude to a cross-Channel assault. Churchill invited him to Chequers, where he personally briefed him on the work being done at Bletchley, to which, as a Commander-in-Chief, he would have access.¹⁷

On 17 May 1943 the British-United States Agreement (BRUSA) was signed between Bletchley Park and the United States War Department. This came to be known as the “written constitution” of Anglo-American crypt-analysis.¹⁸ Its aim was “to exchange completely all information concerning the detection, identification and interception of signals from, and the solution of codes and cyphers used by, the Military and Air forces of the Axis powers, including secret services (Abwehr).” The United States assumed the “main responsibility” for the reading of Japanese military and air codes and cyphers (Magic), the British for reading the German and Italian signals traffic (Ultra). There would be total reciprocity, and total secrecy.¹⁹

Following this agreement, Colonel Alfred McCormack and Lieutenant-Colonel Telford Taylor, from the United States Army’s Special Branch, were sent to Bletchley to see how the system there operated. It was not until September 1943, however, that Churchill finally persuaded Menzies that the BRUSA agreement should be operated without any restrictions, and that the United States Army in Washington should be sent—without restriction—all British Signals Intelligence material, including the Enigma and Geheimschreiber decrypts.²⁰

Riddle of the Balkans

Enigma-based knowledge was continuous, and called for many difficult decisions. The most difficult to confront Churchill and the War Cabinet, and those in receipt of Enigma-based information, related to the Balkans. In the last week of October and the first few days of November 1940, Enigma decrypts had made it clear that the German Air Force was building up facilities for German aircraft use in Romania and Bulgaria, both on the Danube and the Black Sea.²¹ This information was set out in a War Cabinet paper on 5 November 1940.²²

The possibility of a German attack on Greece, with whom Britain had a treaty of alliance, meant that British military, naval and air forces then protecting Egypt from attack across the Western Desert would have to be diverted to Greece. Enigma-based evidence of an attack mounted. At the Defence Committee (Operations) meeting of 8 January 1941, where all present were privy to Enigma, Churchill noted that “all information pointed to an early advance by the German Army, which was massing in Roumania, with the object of invading Greece via Bulgaria.”²³

As a result of assessments being made at Bletchley, this and further Enigma decrypts in the following two days enabled Churchill to inform the Army and Air Force Commanders-in-Chief in the Middle East—General Wavell and Air Chief Marshal Longmore—that “a large-scale movement may begin on or soon after 20th March.”²⁴ Enigma showed that the force that had been assembled for whatever action was planned included two armoured divisions and 200 dive bombers.

Enigma continued to confirm the German threat to Greece. A crucial decrypt on January 9th gave the news that German Air Force personnel were moving into Bulgaria to lay down telephone and teleprinter lines to the Bulgarian-Greek border along the main axis of advance towards Salonika.²⁵ A decrypt of January 18 showed German Air Force hutments being sent to Bulgaria.²⁶ A decrypt two days later showed that the German Air Force mission in Romania was discussing long-term arrangements for the supply of German Air Force fuel to depots in Bulgaria.²⁷

As evidence mounted, a political decision was essential. The War Cabinet’s unanimous conclusion was that Britain must come to the aid of Greece, however hopeless that task might be. With Enigma, the facts were known well in advance, adding to the pressure for a decision. Without Enigma, the dilemma would not have arisen until too late.

German, Hungarian and Italian troops attacked Yugoslavia and Greece on 6 April 1941. By April 30th both nations had been overrun. Between April 24th and 30th, more than 30,000 British, Australian and New Zealand troops were evacuated to Crete, where they awaited a second German onslaught. On the 28th, Enigma revealed the German intention for an air-supported attack, and Churchill telegraphed to General Wavell, Commander-in-Chief, Middle East (then in Cairo): “It seems clear from our information that a heavy airborne attack by German troops and bombers will soon be made on Crete.”²⁸

It would be an entirely airborne attack. But how was the commander on Crete, New Zealand General Bernard Freyberg, who was not privy to Enigma, to be informed without endangering the source? On May 7th, Churchill proposed that Freyberg be sent actual texts of the decrypts relating to German plans for a parachute landing at Maleme Airport in Crete. This was done, but the Secret Intelligence Service warned Freyberg not to act on this information unless he had at least one non-Enigma source for it—lest the Germans realise that their own top-secret communications had been broken. As he did not have any other source, Freyberg was unable to make the necessary dispositions to concentrate his defences at Maleme.

Desperate for Freyberg to see the actual texts of the decrypts regarding German plans for glider and parachute attacks, Churchill wrote to the three Chiefs of Staff on May 10th: “So important do I consider all this aspect that it would be well to send a special officer by air to see General Freyberg and show him personally the actual texts of all the messages bearing upon this subject. The officer would be answerable for their destruction in the event of engine failure en route. No one should be informed but the General, who would give his orders to his subordinates without explaining his full reasons.”²⁹

The airborne landings took place on 20 May 1941. In the fighting that followed, 5326 Allied and 6565 German soldiers were killed; 12,000 British and Commonwealth forces were evacuated from Crete between 28 and 31 May; 12,254 were taken prisoner by the Germans. During the naval actions around the island, eight British warships were sunk. There had been no way to take advantage of the German Enigma messages on land or at sea.

Peril in the Atlantic

In the summer of 1940 the cryptologists at Bletchley had broken the settings of the German Home Waters Naval Enigma, but these settings were changed at the end of 1940. From that moment, sinkings by German submarines were heavy and continuous, even reaching the Atlantic coast of Canada and the United States.

A breakthrough in Bletchley’s ability to read the German Naval Enigma came with the Lofoten Islands raid on 4 March 1941. Proposed by Hugh Dalton, the head of Special Operations Executive (SOE), and approved by Churchill and the Chiefs of Staff, the raid—Operation Claymore—was much boasted publicly as having been for the destruction of the fish oil processing plant there. This was the most important source by which Germany obtained vitamins A and D: indeed, during the raid, eleven factories and 800,000 tons of fish oil were destroyed.

The actual aim of the raid, kept secret throughout the war and for many years after 1945, was to capture an Enigma machine used by the German Navy, the code keys of which had been proving virtually impossible to break. One such Enigma machine was on board the German trawler Krebs. Its commander, Lieutenant Hans Küpfinger, had managed to throw his machine overboard before he was killed. He had insufficient time, however, to destroy other elements of the Enigma message procedure, including his coding documents.

After three weeks’ intensive work at Bletchley, it became possible for British Intelligence to read all German naval traffic in Home Waters for the last week of April and much of May, with a relatively short delay of between three and seven days. By July the German naval Home Waters messages were being read with a maximum delay of seventy-two hours, and often with a delay of only a few hours.

On 6 April 1941, Churchill informed Roosevelt of a British naval success in the Mediterranean: A convoy of five German and Italian supply ships loaded with ammunition and transport vehicles was sunk, along with three Italian destroyer escorts, with the loss of only one British destroyer. The enemy supply ships were carrying units of the 15th Panzer Division to North Africa. The engagement constituted the first significant victory for Bletchley’s decrypting of the German Air Force Enigma in the Mediterranean shipping war. Henceforth, and with increasing impact, German and Italian high-grade decrypts were to contribute significantly to the sinking of Axis supply ships.

This Enigma window into German naval operations came, however, to an abrupt end. At the end of January 1942 the U-boats, which had hitherto used the same Enigma machine as other ships and authorities both in German home waters and in the Atlantic, acquired their own form of Enigma machine, unique to them. Suddenly, the U-boat’s most secret signals, which had been so carefully monitored at Bletchley since the summer of 1941, became unreadable once more, and were to remain unreadable for nearly a year, followed by a further six months irregularities and delays in decrypting.

This had dire consequences. In April 1941 more than seventy British merchant ships had been sunk; in May 1941 more than ninety; in June, sixty. By the end of July 1941, however, as a result of remarkable progress at Bletchley Park, all German U-boat instructions were being read continuously, and with little or no delay. As a result, transatlantic convoys could be routed away from the U-boat packs. For the rest of 1941, one major area of Churchill’s anguish was calmed.

By the end of August, British naval sinkings were further reduced by another cryptographic success, the breaking of the Italian naval high-grade cypher machine, “C.38m.” The first intelligence transmitted from Bletchley Park to the Middle East from this new source was sent to Cairo on June 23rd, giving details of the sailing of four liners from Italy to Africa with Italian troops.

In the ever-fluctuating fortunes of the war at sea, Churchill was kept fully informed of the secrets that Enigma revealed. Sometimes an Enigma message could pinpoint a British setback. On 3 September 1941, Churchill was shown an actual decrypt, which read: “Tanker Ossag to leave Benghazi evening 3rd September after discharging cargo of aviation fuel.”³⁰ Distressed that the tanker had not been attacked while on its way to Bengazi, Churchill noted that Admiral Cunningham, Commander-in-Chief, Mediterranean Fleet, “should feel very sorry about this. It is a melancholy failure.”³¹

On 16 February 1942, German Enigma messages revealed to Churchill that the three German warships which had escaped from the Atlantic port of Brest through the English Channel to their home ports had been badly damaged during their transit. He was unable to reveal this, although he was much criticised in Parliament and the press for allowing the ships to escape unscathed. As Churchill warned Roosevelt, “We cannot dwell too much on the damage they sustained”—damage, Churchill added, “you may have learned from most secret sources.”³²

From the U-boat menace, the nightmare of heavy sinkings returned on 1 February 1942, when a new Enigma machine came into service using a special cypher for the Atlantic and Mediterranean U-boats. The new cypher remained unbroken for more than ten months, until an Enigma machine was recovered with all its rotors.³³ This was a turning point for the naval codebreakers in Hut 8 at Bletchley, who passed on the new decrypts to the Naval Section in Hut 4, for translation, intelligence extraction and transmission to Brigadier Menzies, the Chief of the Naval Staff and Churchill.

The new German Naval Enigma came into being after—on at least two occasions—Allied success against the U-boat operations led Vice-Admiral Karl Dönitz, then Commander of the German Submarine Force, to investigate a possible security breach as the reason. Two breaches were considered most seriously: espionage, and Allied interception and decoding of the German Naval Enigma. Two investigations into communications security came to the conclusion espionage was more likely, unless there was a third reason, that the Allied success had been accidental. Nevertheless, as a precautionary measure, on 1 February 1942, Dönitz ordered his U-boat fleet to use an improved version of the Enigma machine—the M4—for communications within the Fleet.

The German Navy—of which Dönitz became the Commander-in-Chief in 1943—was the only branch of the German armed services to use this improved version; the German Army and Air Force continued to use their existing versions of Enigma. The new system, known to the Germans as “Triton,” was called “Shark” by the Allies.

This setback caused by “Shark,” coming at a time when the U-boat fleet was rapidly increasing in size, coincided with a move of the U-boats to what Churchill called “American Waters”: the Atlantic west of the 40th Meridian, and the Caribbean. U-boat successes there for six consecutive months were largely responsible for an alarmingly sharp rise in the monthly rate of Allied merchant ship losses, especially of oil tankers. When, in the late summer of 1942, the U-boats turned once more against the Atlantic convoys, these losses were to reach unprecedented levels.

This second loss of the ability to decrypt top-secret German naval signals brought Britain’s food and war materials lifeline almost to a halt. For Churchill, as for those in the know about the full extent of the sinkings and the failure to read Enigma, this was a grave worry. On the evening of 18 November, Churchill presided at the first meeting of the newly established Anti-U-boat Warfare Committee, charged with finding some means to meet the relentless challenge of German submarine successes.

In October that same year, U-boats had sunk twenty-nine Allied ships in convoy and fifty-four ships sailing independently. In November the figure rose to thirty-nine ships in convoy, and seventy ships sailing independently. The total tonnage of Allied shipping lost to U-boat attack in November 1942 was 721,700 tons, the highest figure for any month of the war.

Turning the Tide

The reason for these U-boat successes was Bletchley’s inability to read the German Enigma key used in its U-boat communications. Then, on 30 October 1942, British destroyers attacked the German submarine U-559 on the surface in the Eastern Mediterranean, seventy miles off the Egyptian coast. While U-559 was sinking, two Royal Navy men, Lieutenant Tony Fasson and Able Seaman Colin Grazier, from the destroyer Petard, seized two vital Enigma codebooks. Unable to escape the sinking submarine, they managed to pass the codebooks to Canteen Assistant Tommy Brown. For their actions, Fasson and Grazier were each awarded the George Cross; Brown received the George Medal, the medal’s youngest recipient; it was discovered that he had been under age when he enlisted.³⁴

It took three weeks for the codebooks to reach Bletchley Park. When they did, the effect was dramatic. On 13 December 1942, a Sunday, the codebreakers in Hut 8 worked with as much intensity as they had ever worked to try to crack the M4 “Shark” cypher used by the Admiral Dönitz to communicate with his U-boats in the Atlantic. By midday, solutions of the four-rotor Enigma U-boat key began to emerge. During the afternoon, Hut 8 telephoned the Submarine Tracking Room at the Admiralty to report the breakthrough. Within an hour of this news, the first intercept came through: making known the position of fifteen U-boats in the Atlantic. Other intercepts arrived in a continuous stream until the early hours of the next morning. The Admiralty’s Submarine Tracking Room was once more able to route British convoys away from German U-boat concentrations.

As a result of this triumph of cryptography, in December 1942, the number of Allied vessels sunk by the U-boats fell to nineteen in convoy and twenty-five sailing individually. In January 1943 the figures had dropped to fifteen in convoy and eighteen independently, as a result of successful evasive routing of the known and located dangers.

In the Battle of the Atlantic, after a disastrous year, the fortunes of war had turned decisively in favour of the Allies. On 2 May 1943, Bletchley decrypted a telegram from General Oshima, the Japanese Ambassador in Berlin, reporting to Tokyo that Hitler, while hopeful of a renewed U-boat offensive, had complained to him that because the war had started too soon, “we have been unable to dominate the seas.”³⁵ Two days later, in a lengthy battle, two U-boat packs attacked and sank twelve merchant ships in convoy (there were forty-one U-boats in the two packs, seventy merchant ships in the convoy).

This German success, the last on such a scale, was only achieved with the loss of seven of the U-boats. Not only did Enigma decrypts in the coming days confirm this loss, but also gave evidence of the growing U-boat fears both of Allied aircraft and of surface escorts.

Studying the Enigma decrypts relating to the Mediterranean, Churchill learned from German sources of the success of Bomber Command attacks on the Port of Tunis. Using the word “Boniface” to imply an Allied agent rather than Germany’s own top-secret signalling system, Churchill telegraphed to Air Marshal Tedder on May 2nd: “Boniface shows the decisive reactions produced upon the enemy, and that the little enemy ships are also important.”³⁶

These “little ships” were those in which, as the Enigma decrypts showed, supplies of fuel and ammunition were still reaching Tunis. Larger ships, being more visible from the air, were finding it impossible to get through.

In the Mediterranean on May 4th, as a result of an Enigma decrypt, British destroyers sank the Campobasso off Cape Bon, and on the following day, also alerted by Enigma, the United States Air Force sank the 6,000-ton San Antonio: the last large merchant ships to try to reach Tunis. As Churchill had foreseen, the Germans continued to try to use smaller craft, and even planned—as one Enigma decrypt showed on May 6th—to use U-boats to ferry fuel.

On July 8th, Churchill was able to telegraph to Stalin that in seventy days, fifty U-boats had been sunk.³⁷ On the 14th, he reported to Roosevelt that seven U-boats had been sunk in thirty-six hours, “the record killing of U-boats yet achieved in so short a time.”³⁸ Guided by Enigma, the Battle of the Atlantic had been won by the Allies. Sixty British merchant ships had been sunk in March, thirty-four in April, thirty-one in May and eleven in June; the June figure confirmed that it was safe to release warships and merchant ships for all theatres of war, including the landings in Sicily, Italy and Normandy.

Churchill could offset the few failures at sea with many successes. Enigma helped to seal the fate of the German battleship Bismarck in May 1941, when 2300 German sailors perished, and the battle cruiser Scharnhorst in December 1943, when 1995 of her crew of 2200 were killed.³⁹ Another naval challenge, the attempt to destroy the German battleship Tirpitz—the largest battleship ever built in Europe—as she sheltered in Norwegian waters, was reflected in many German Air Force Enigma messages: one, in April 1942, revealed German appreciation of a “most courageous” but ineffective attack that month by Bomber Command.⁴⁰ Enigma played its part in the final, successful attack on Tirpitz by Bomber Command on 12 November 1944, when the pride of the German Navy capsized, and 1000 of her crew of 1700 were drowned.

Endnotes

1. War Office papers, WO 199/911A.

2. “German Preparations for Invasion”, 4 and 9 October 1940, War Office papers, WO 199/911A.

3. “Officer Only.” “Most Secret.” “Invasion of Britain,” MI14, 12 January 1941: War Office papers, WO 119/911A.

4. Secret Intelligence Service papers, series HW1.

5. Minute of 16 October 1940: Churchill papers, 20/13.

6. Letter of 22 September 1941: Secret Intelligence Service papers, HW1/86.

7. John Colville, diary entry, 4 May 1941: Colville papers.

8. John Martin papers and conversation with Martin Gilbert, 1979.

9. “Secret and Confidential.” “Prime Minister Only.” 21 October 1941: Hinsley and others, British Intelligence in the Second World War, volume 2, (HMSO: London 1981), 655.

10. “Action This Day.” Hinsley and others, 657.

11. Dir/C Archives, No. 2592: Hinsley and others, 647.

12. Ralph Erskine and Peter Freeman, “Brigadier John Tiltman: One of Britain”s finest cryptographers,” Cryptologia, Volume 27, Issue 4, October 2003.

13. Churchill’s comments on the Enigma decrypts are in the Secret Intelligence Service papers, reference HW1.

14. Office of Chief of Naval Operations (OPNAV): 20th Division of the Office of Naval Communications, G Section/Communications Security, was the U.S. Navy signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications from Japanese, German, and Italian navies. In addition, OP-20-G copied diplomatic messages of many foreign governments. Its branches included G-30 (Pacific Theater), G-40 (Atlantic Theatre), G-70 (Clandestine), G-80 (Strategic Information Coordination), GI-A (Correlation and Dissemination, Atlantic), GI-D (Correlation and Dissemination, Diplomatic), GI-P (Correlation and Dissemination, Pacific), GT (traffic analysis) and GY (cryptanalysis).

15. “Most Secret.” 8 February 1942: Churchill papers, 20/52.

16. U.S. National Security Agency, Central Security Service, “Solving the Enigma”: http://www.nsa.gov/publications/publi00016.cfm.

17. Stephen Ambrose, Ike’s Spies (Jackson, Mississippi: University Press of Mississippi, 1981), chapter 1.

18. Bradley F. Smith, The Ultra-Magic Deals and the Most Secret Special Relationship 1940-1946 (New York: Presidio, 1993), 153.

19. The complete text of BRUSA was kept secret until November 1995, fifty years after the end of the war. It was published in full by John Cary Sims, “The BRUSA Agreement of May 17, 1943,” Cryptologia, Volume 21, Issue 1, January 1997.

20. Ralph Erskine and Peter Freeman, “Brigadier John Tiltman: One of Britain’s Finest Cryptographers,” Cryptologia, Volume 27, Issue 4, October 2003.

21. Decrypt CX/JQ 417, 1 November 1940, revealed German plans to install aircraft warning systems in Romania and Bulgaria.

22. War Cabinet Paper No. 431 of 1940, 5 November 1940, “Possibility of Enemy Advance through the Balkans and Syria.” Cabinet papers, 70/1.

23. Defence Committee (Operations), 8 January 1941: Cabinet papers, 69/2.

24. Telegram of 11 January 1941: Churchill papers, 20/49.

25. F.H. Hinsley and others, British Intelligence in the Second World War, Its Influence on Strategy and Operations, (HMSO: London, 1979), I: 353.

26. Decrypt CX/JQ 603.

27. Decrypt CX/JQ 605.

28. Telegram of 28 April 1941, telephoned to Downing Street on a secure line from Chequers: Premier papers, 3/109.

29. “Most Secret” minute of 10 May 1941: Premier papers, 3/109.

30. Enigma decrypt CX/MSS/205/T2 of 3 September 1941.

31. 3 September 1941: SIS papers, HW1/43.

32. Prime Minister’s Personal Telegram No.257 of 1942, 16 February 1942: Churchill papers, 20/70.

33. For details of the ten Enigma machines recovered at sea see “The Capture of German Enigma Machines and Codebooks, 12 February 1940 – 4 June 1944” in Martin Gilbert, Atlas of the Second World War (New York: Routledge, 2008), Map 204.

34. Tommy Brown died later in the war, in Britain, trying to rescue his two sisters from a burning building.

35. Secret Intelligence Service archive, series HW1.

36. “Personal,” “Most Secret,” Prime Minister’s Personal Telegram No. 630 of 1943, 2 May 1943: Churchill papers 20/111.

37. “Personal and Most Secret,” Prime Minister’s Personal Telegram No. 975 of 1943, 8 July 1943: Churchill papers, 20/114.

38. “Personal and Secret,” Prime Minister’s Personal Telegram No. 1031/3 of 1943, 14 July 1943: Churchill papers, 20/115.

39. “Sink the Bismarck” was Churchill’s instruction following the sinking of the British battle cruiser Hood by Bismarck on 24 March 1941, with 1415 deaths (and only three survivors). During the sinking of Scharnhorst three days later, the Staff Officer (Intelligence) on board the battleship Duke of York was Edward Thomas, later one of the historians of British Signals Intelligence.

40. Decrypt 0011 of 29 April 1942.