Intelligence in War: The Value--And Limitations--Of What the Military Can Learn About the Enemy (37 page)

The third period of the Battle of the Atlantic began in April 1941, the month after Winston Churchill coined the phrase “Battle of the Atlantic.” Dönitz had by then assembled enough trained U-boat crews to begin organising patrol lines and wolfpacks in the central North Atlantic, though his boats had been driven into those deeper waters, away from the sea approaches to the British Isles, by the increasing numbers of Royal Navy escorts and Coastal Command aircraft. Sinkings rose but the Admiralty also had considerable success during the year in routing convoys away from U-boat patrol lines, thanks to Bletchley decrypts. In September 1941, for example, when thirty-two U-boats were on patrol in the North Atlantic, twelve sank no ships at all, and only four sank more than 10,000 tons, or two ships each.

Dönitz’s prospects changed suddenly with the inception of the fourth period in January 1942, when he was able to withdraw his U-boats from the central North Atlantic, break up his packs and patrol lines and deploy individual boats, under now often highly experienced skippers, against the coastwise shipping of the United States on its east coast and in the Caribbean. U-boat captains described the next six months as their “Happy Time.” Targets were numerous, so were sinkings. In January 26 U-boats operating in American waters sank 400,966 tons of shipping, 71 cargo ships or tankers, for no losses at all. February was worse proportionately: 18 U-boats sank 344,494 tons, 57 ships. In April, after a very bad March when 406,046 tons were sunk, 31 U-boats sank 133 ships of 641,053 gross tons; and so the awful summer went on. By the end of August, when the Americans at last instituted proper anti-submarine measures, 609 ships, of 3,122,456 gross tons, had been sunk, for the loss of 22 U-boats, out of 184 engaged.
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The extent of Dönitz’s success was due to the refusal of the United States Navy to institute convoy at the outset, in a bizarre repetition of the British Admiralty’s policy of 1914–16. Admiral Ernest King, Chief of Naval Operations, formed the view that weakly escorted convoys would merely provide more plentiful targets than individually sailed ships, and so left America’s coastwise traffic to its fate. In his defence, it is argued that he used such warships as he could muster in Atlantic waters to escort the convoys taking American troops to Britain, and that not one suffered loss; it was also the case that he was meanwhile waging a life-and-death struggle with the Japanese navy in the Pacific, which consumed almost every serviceable warship his navy possessed. Nevertheless, there was undoubtedly an American anti-convoy prejudice, as evidenced by the U.S. Navy’s organisation, as by the Admiralty in the First World War and again at the outbreak of the Second, of U-boat “hunting” groups which, as reason should have taught and experience did teach, found few if any U-boats to attack. By 1941 the Royal Navy was wholly committed to the correct view that, if U-boats were to be found and sunk, they had to be presented with targets to attack that could defend themselves, in short, convoys with strong close escorts.

The conclusion of the American Happy Time confronted Dönitz with the need to risk his U-boats against such targets again. The inception of the period that ensued, the fifth and climactic stage of the Battle of the Atlantic, from September 1942 until May 1943, ushered in a dreadful episode in sea warfare, marked by heavy losses of merchant ships and tragic loss of life, all suffered in North Atlantic weather at its worst. Nevertheless, the crisis can be seen in another light. It was the moment in the maritime conflict between the
Kriegsmarine
and its opponents—the Royal Navy, the Royal Canadian Navy, the United States Navy and their associated air forces—when Dönitz was forced, in the classical terms of sea warfare, to give battle. He had argued throughout his life as a professional naval officer that there was a victory waiting to be won between a surface fleet and its submarine enemy. At the end of 1942 he was challenged to win such a victory—and lost.

The part played by Bletchley Park in that victory, though crucial, was complex. Its struggle was two-sided, not unilateral, for the German navy had its own interception and decryption service—the
Beobachtungs
(Observation)
dienst,
known as the B-dienst—and it had a record of considerable success against the Royal Navy’s disguised transmissions. Because of Britain’s efficiency in breaking into Germany’s codes during the First World War, an undeniable complacency prevailed in the Naval Intelligence Department during the post-war years and well into the Second World War. The British believed that decryption was a one-way traffic even if, at first, they were unable to break Enigma. Because also they had unwisely trumpeted their decoding achievements of 1914–18, they had put the Germans on their mettle so that, long before the outbreak of war in 1939, the B-dienst was breaking the then current Admiralty code, a system of five-digit groups called the Naval Code, which was super-enciphered mathematically. Through carelessness, the more secure Naval Cypher was also betrayed to the enemy. It was a familiar story; a cipher officer used the super-encipherment book of the Naval Cypher to super-encipher messages sent in the Naval Code. As the latter could be read, the former was quickly broken and was read currently and continuously until 20 August 1940.
¹⁸

The English-language department of the B-dienst employed 900 people before the war; the number would rise to 5,000 by 1942. The B-dienst was located at German naval headquarters in Berlin and was led by Wilhelm Tranow, a radio technician who had first been employed to test the security of Enigma. The security of their ciphers was a matter of deep concern to the Germans throughout the war. It was constantly reviewed, as their own was by the British. Both navies remained convinced, nonetheless, that they could not be overheard, the Germans with far better reason. They correctly reasoned that, even were the British able to acquire three of the four elements of the Enigma system—the machine, the setting list, the indicators and the tables of bigrams which designated the grid-squares on the oceanic chart—they would still not be able to read messages. They discounted the possibility of the British acquiring all four and, as the result of a prolonged investigation during 1942, instituted new precautions against the operators’ resorting to short-cuts. More to the British disadvantage, they also altered the U-boats’ Enigma machines to accept a new, fourth rotor which, in combination with an adapted reflector, multiplied by twenty-six the number of possible keys.
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The result was that between 1 February 1942 and the following December, Bletchley lost its way into Enigma altogether, with a calamitous impact on sinkings. The effect was heightened by a sudden German breakthrough into British naval codes. Suspecting, correctly, that both Naval Cypher No. 2, the successor to the Naval Code, and the first Naval Cypher had been penetrated, as the latter had been since September 1941, the Admiralty had introduced in December 1941 the new Naval Cypher No. 3, still a super-enciphered code, not a cipher proper. It worked by the traditional method, the adding of numbers, from a book of number groups, to the groups indexed in the main code (“Cypher”) book and was decoded by subtraction. The books were issued to the Royal, Royal Canadian and United States Navies for use in the passage of convoys across the North Atlantic.

During January 1942 the B-dienst succeeded in reconstructing both the codebook for Naval Cypher No. 3 and the subtraction tables used with it. In consequence, it was able to read 80 per cent of the convoy traffic, often twenty to thirty hours in advance of the movements signalled. This warning allowed ample time for U-boats to be positioned in a convoy’s path, since the difference between the speed of an average convoy, seven to eight knots, and that of a surfaced U-boat, at least sixteen knots, meant that patrol lines and wolfpacks moved at twice the speed of their prey. In twenty-four hours, during which a convoy advanced 180 miles, a hunting U-boat could move 360 miles to cut it off, submerging only just before the moment of contact.

U-boat commanders were trained, according to Dönitz’s pre-war experiments with torpedo boats, to linger at the limit of visibility, until the fall of dark, on the convoy’s projected line of advance, then to surface, if possible within the convoy columns, fire salvoes from bow and stern tubes simultaneously, and to submerge and make their escape as the escorts appeared. Depending upon how much disruption had been caused in the convoy pattern, a second attack might be mounted.

The difficulty for the U-boats was to locate a convoy in the first place. The limit of visibility at best from a conning tower was ten miles; a patrol line of ten U-boats could therefore cover 220 miles of sea. Dönitz attempted to extend the area of sweep by securing the services of 1/KG40, a squadron of long-range Condor aircraft, from the Luftwaffe; but the growing efficiency of British air patrols in 1941–42, which had driven the U-boats into the central Atlantic, also put them beyond the Condors’ range; an effort to fly an autogiro on a cable from U-boat conning towers proved as impractical as it was dangerous. A convoy of fifty ships, meanwhile, occupied a front of only 2,400 yards. In the enormous spaces of the Atlantic—at least nine million square miles of operational waters—the area occupied by a convoy and the space covered by a questing U-boat patrol line were both relatively tiny. The one could be missed by the other with the greatest ease and usually was. Between 1 January and 31 May 1943, for example, the height of the Battle of the Atlantic, forty-eight out of eighty-six convoys sailed were not found by U-boats at all.
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Bad weather played a part, hiding the convoys from German eyes or forcing U-boats to seek shelter from the elements below periscope depth; so did routine alterations of course and emergency turns if contact were made. Deliberate rerouting of convoys, however, away from patrol lines and wolfpacks, located by Bletchley, was the most productive method. Indeed, it was Hut 8’s main task to provide such intelligence, which was Bletchley’s principal contribution to winning the Battle of the Atlantic and so, arguably, to assuring that the war would not be lost.

David Kahn, the great historian of cryptography, gives a dramatic account of one such rerouting contest in his book on the U-boat war,
Seizing the Enigma
. It describes the progress of Convoy SC127—so code-named for the point of departure from Sydney, Cape Breton Island at the mouth of the St. Lawrence River in Canada—towards Liverpool in England in April 1943. The direct distance, measured from Halifax, Nova Scotia, was about 4,000 miles. The planned course, including changes of direction, was longer. The course actually sailed, around identified U-boat traps, was longer still.

SC127 comprised more than fifty ships, arranged in thirteen columns, escorted by five Canadian warships. It first steered east, at about 7.5 knots, then turned slightly northeast towards a spot in the ocean designated Point F by the Admiralty’s Trade Movements Section. On 16 April, when it departed, both Bletchley and OP-20-G in Washington were decrypting Dönitz’s radioed instructions to his U-boats, and their encrypted reports, at a delay of three days. Both tracking organisations knew, however, that he had over sixty boats in the Atlantic (in fact sixty-three) and that twenty-five were located on SC127’s path due east of the Gulf of St. Lawrence. They formed a patrol line 650 miles long, aligned north-west–south-east, due south of Cape Farewell, the southernmost point of Greenland.

On 18 April Allied cryptanalysis—at either Bletchley or Washington or both—broke Dönitz’s transmission of the 17th, ordering the formation of the line code-named Titmouse. Though it referred to the U-boats involved only by their captains’ names, a new security measure, the key was one that had been solved; less clear were the boats’ locations, since neither Bletchley nor OP-20-G had comprehensively established the inner encryptions of the grid-square designations. Moreover, the B-dienst was reading correctly Naval Cypher No. 3 and so knew, from Admiralty transmissions to SC127, that it was aware of the existence of Titmouse.

Dönitz nevertheless, with some complacency, decided that SC127 would maintain its current course; he may have done so because he was also tracking the progress of the convoy following SC127, HX234, which had just made a sharply evasive change of course, and perhaps concluded that the British would not order two convoys simultaneously to divert from their planned line of advance, trusting to HX234’s alteration to distract Titmouse from SC127. In this he was wrong. On 20 April the Convoy and Routing Section in Washington, the U.S. Navy’s equivalent of the Admiralty Trade Division, ordered SC127 to make a radical change of direction, just before it reached the designated Point F due east of Cape Race in Newfoundland. Instead of continuing north-eastward it was to turn almost due north, leaving the coast of Newfoundland to port, and so evading by several hundred miles the grid squares in which the twenty-six U-boats of Titmouse were lying in wait for it.

Alerted by Titmouse’s failure to find SC127, Dönitz now formed another patrol line, code-named Woodpecker (bird names were currently in fashion at U-boat headquarters), and deployed it to the south, wrongly guessing that SC127 had gone in that direction. SC127 therefore proceeded untouched on its leisurely way. Two factors intervened to spare it. One, unfortunately, was that Titmouse had found HX234 and was beginning to sink ships; the sinkings would be sustained by yet another hastily formed wolfpack, Blackbird. The other was that because of SC127’s northward diversion, in one of the worst winters of the twentieth century which brought ice as far south as Newfoundland, it began to run into bergs and floes that forced it to change course yet again, into the Denmark Strait between Greenland and Iceland. The danger from ice also caused the convoy to slow down, thus throwing out German presumptions about its rate of progress towards other U-boat concentrations further east in the central Atlantic.