The Allies—the British and Canadians first, the Americans later—enjoyed superior tactical intelligence throughout the Atlantic battle. They had sonar (Asdic) from the start, HF/DF and centimetric radar from 1942, aerial surveillance—of expanding range—from 1939. There were serious lapses, associated with shortages of escorts in 1939–41 and with American refusal to form convoys in the first six months of 1942. Nevertheless, while tactical intelligence collected by the U-boats was limited by a patrol line’s line of vision, supplemented eventually and only close to France by that of the Condor aircraft, the convoy escorts and their associated patrol aircraft always had the edge, which increased throughout the war. Eventually the Allies’ tactical advantage became overwhelming.
The question of the importance of intelligence in the Atlantic battle therefore turns on the strategic issue: B-dienst versus Bletchley. The B-dienst was a remarkable organisation. Working largely with human resources, rather than the electromechanical devices later available in England and the United States, the German decoders—their target was the Naval Cypher No. 3, a book code mathematically super-enciphered—achieved impressive success for several long periods. Having broken the old (Royal) Naval Code, a system of five-digit groups, as early as 1935, it used the clues the Code provided to attack the newer Naval Cypher and by April 1940 were reading as much as 30 per cent of British transmissions. The Naval Cypher and Code were replaced, however, in August 1940 by new versions of each, against which the B-dienst had less success. It recovered its form when Naval Cypher No. 3 was introduced in June 1941 for transatlantic communication, and read it throughout 1942; in December it was reading 80 per cent of messages sent. Between 15 December, when the British added precautions, and February 1943, it was again in the dark but then found a way back, “sometimes reading directives to convoys ten and twenty hours before the movements they ordered took place.” Not until June, when an entirely new cipher was distributed, were British signals at last made secure.33
On their side, the British, and later the Americans of OP-20-G in Washington, experienced similar periods of light and dark. The British benefited from a series of captures during 1940 and 1941—of U-33 in February 1940, of the patrol boat VP2623 in April and, in March 1941, of the trawler Krebs during the commando raid on the Norwegian Lofoten islands. In May and June the weather ships München and Larenburg were seized in deliberate “cutting out” operations, while on 9 May, U-110 fell into British hands. Each yielded some material—either parts of the Enigma machine, or gridded charts, or enciphering material—which, when added to what Bletchley had been able to reconstruct from interceptions, furthered decryption. Traffic for February, May, June and July 1941 was read in part or whole, as a result, and some in real time. After August 1941 until February 1942, the Heimisch key, called Dolphin at Bletchley, was read at a delay of not more than thirty-six hours. The decryption was considerably assisted by the deployment of the first bombes, which greatly assisted in the identification of possible Enigma key settings. Much help was also provided by the carelessness of German operators in retransmitting Enigma messages previously enciphered in the dockyard (Werft) or weather ciphers, which were quite easily read.34
After February 1942, however, Bletchley lost its way into the U-boat traffic because of the German adoption of a new Enigma key, called Shark by the British, Triton by the Germans, on the first of the month. Regular reading in real time did not resume again until December 1942. This period coincided with the reciprocal B-dienst success against British naval ciphers and with a peak of German advantage in the Battle of the Atlantic. In June (129), July (136) and August (117), monthly sinkings exceeded one hundred ships. Total sinkings exceeded four million tons.35
Paradoxically, however, during what historians of the U-boat war denote as “the great convoy battles” in the first five months of 1943, the British were frequently reading Enigma in real time and the Trade Division was redirecting some convoys away from U-boat patrol lines. Moreover, although there were massacres, U-boat losses were also rising, eventually to unbearable levels, forcing Dönitz, in May 1943, to take his boats out of the North Atlantic and so, effectively, admit defeat.
What, therefore, are we to make of F. H. Hinsley’s assertion that Bletchley achieved one of its undeniable successes in the Atlantic battle? Hinsley, a Bletchley veteran, is notably modest in the claims he makes for the British Government Code and Cipher School. He specifically rejects the view that “Bletchley won the war,” and rightly so. His estimate of Bletchley’s success in the war against the U-boats must be set, however, against Clay Blair’s sober and heavily documented assessment that over 99 per cent of all ships forming transatlantic convoys reached their destinations safely. Out of 43,526 ships sailed (many several times, of course), 272 were sunk by U-boats. Many others were sunk but usually when sailing independently or having left convoys, either to “straggle” (fall behind) or to “romp” (sail ahead).
Allied merchant seamen paid a terrible price. Over 30,000 out of 120,000 in the British Merchant Navy alone were killed in the struggle against the U-boats. The U-boat crews, however, suffered worse: 28,000, out of an enlisted force of 40,000, died in the destruction of their submarines—which amounted to 713. Aircraft sank 204, warships 240, aircraft flying from escort carriers 39; ships and aircraft operating together sank 84; mines, accidents and other incidents—such as U-boats ramming each other—made up the difference in numbers.36
The outcome of the Atlantic battle, seen in perspective, suggests that intelligence, as in so many other operational circumstances, was, though significant, secondary to the age-old business of fighting the issue out. In easy times, as during the U-boats’ Happy Time along the eastern American seaboard in the first six months of 1942, it was not strategic intelligence but day-to-day happenstance that yielded the victims. In the more difficult times, particularly during the “great convoy battles” of early 1943, Bletchley’s ability to steer convoys out of danger frustrated many of the traps Dönitz laid, but it was the stolid endurance of the merchant seamen, sticking to convoy commodores’ orders, and the dogged determination of the escort crews to fight back, that did the U-boats in. Modern interpretations of the Battle of the Atlantic represent it as a true battle, in which one side attacked, the other accepted the challenge to defend or counterattack, and counterattack prevailed. That seems correct. The Battle of the Atlantic could have been won without the assistance of the codebreakers, greatly though they helped to tip the balance in the favour of the defenders.