The top-secret breaking of the German Enigma code by Alan Turing, and the codebreakers working with him at Bletchley Park, was one of the greatest British coups of the second world war. It helped ships delivering vital supplies to the UK during the darkest days of the war to evade the packs of German U-boats trying to hunt them down, and enabled Britain to rebuild its strength and re-equip its armies in preparation for its bid to expel the Nazi armies from Europe.
Now extraordinary fresh details can be told of how the Royal Navy seized vital cipher information from captured German boats to make the work of the codebreakers possible.
The Enigma machine did not actually send the messages. It was used to transform normal German into gibberish which was then transmitted using morse code over the airwaves. British intercept stations could listen in to these signals, but because they were encoded, they could not understand what was being said.
The British capture of a string of German vessels – and their Enigma machines and codebooks – during the first seven months of 1941 changed all that. Using the items seized, Alan Turing and his fellow codebreakers were at long last able to work out how to read Germany’s naval Enigma messages. But there was a glitch. Every now and then the Germans, suspecting that their code might have been compromised, altered it, blacking out the codebreaking effort. The longest blackout occurred following the German order that vessels operating in the Atlantic and Mediterranean after 1 February 1942 should insert a fourth rotor into their machines. Previously they had only used three.
This had disastrous consequences for Britain and her allies. While the naval Enigma messages were being read, convoys could be routed clear of the Nazi wolf packs lying in wait in the Atlantic. At a stroke this safety net had disappeared. From February to October 1942 hundreds of thousands of tons of allied shipping was sunk each month. There was a growing fear that Britain might eventually be starved into submission.
The gloom was only lifted after the seizing of a U-boat, U-559, with her codebooks on 30 October 1942, 75 years ago, enabled Bletchley Park to break the code once again. It is this game-changing capture whose anniversary will be celebrated at the end of this month.
When I did the original research for my Enigma book, the available evidence suggested that the seizing of the codebooks was all down to a lucky break. Documents declassified more recently reveal that in fact a conscious effort was made to train British destroyer commanders so that they could extract as much cipher material as possible from captured vessels.
The only luck involved on the British side when a U-boat was finally cornered was the identity of the commander of the destroyer on the spot. It was Mark Thornton, a 35-year-old lieutenant commander, who had become obsessed not only with making his ship, HMS Petard, one of the best run in the Navy, but also with the desire to capture a U-boat and its codebooks.
A thickset, stocky man with a huge head set on powerful shoulders and the features of a boxer, the seeds of his fearsome reputation were sown on his very first day as the commander on the Petard. He told his assembled crew that his war experience to date had proved that his methods made him indestructible, and that while he was their leader they must adopt them too.
He backed up his promise to protect them by introducing training methods which, while effective, might have been described today as abusive. It was perfectly reasonable for him to insist that his crew should always be on the lookout for submarines. However to ensure they complied, he would climb up into the ship’s crow’s nest and pelt those he saw slacking on the deck below with pebbles, pieces of chalk and sometimes even with teacups.
On one occasion he let off a firecracker in the men’s sleeping quarters and then had a fire hose trained on his men as they rushed from their hammocks to their action stations. On another occasion he ordered his officers to climb out of a wardroom porthole during a gale so that they could swim around the stern of the ship and climb in through a porthole on the other side of the room. His order was only countermanded after a senior officer refused to obey his instructions on the ground that compliance would be tantamount to committing suicide.
The wardroom chef collapsed and died during one simulated exercise. His corpse was thrown into the sea. It was a miracle other men were not washed overboard whenever Petard left harbour. Thornton would turn the ship into the waves with such ferocity that they would completely cover the men dealing with securing the anchor, who had to hang on for dear life.
Such behaviour led some to wonder whether Thornton was mad, a view which was strengthened by his habit of getting up during meals and pummelling the bulkhead with his fists, shouting: “I must have action with the enemy now!” He was certainly eccentric. When he was seen firing a Lewis gun at a flock of gannets, he yelled at his men that he could not bear the sight of the murderous birds who were robbing the sea of its fish.
It is no surprise to find that, drilled as they were by such bullying tactics within the suffocating confines of a ship from which there was no escape, the officers and crew would go to almost any lengths to please him, or to get him off their backs.
But Thornton’s striving for perfect efficiency would not have been enough on its own. There would have been no dividend had it not been for systemic German inefficiency on their U-boats. According to Hermann Dethlefs, a 19-year-old trainee officer serving on U-559, his German commander was not paying attention shortly after midday on 30 October 1942 when the U-boat, which was searching for convoys in the Mediterranean between Port Said, Egypt and Haifa, Palestine, was spotted near the surface from a British plane.
The U-boat commander gave the order to dive, but from that moment on, British planes and the five destroyers including Petard summoned to the spot never lost touch with the German submarine for long.
The many depth charges dropped did not hit the U-boat, but the water that as a result of the explosions leaked in, led to the stern of the U-boat sinking lower than her bow.
In an attempt to rebalance the vessel, all those who could were ordered to go to the front. “I went too,” Dethlefs remembered. “We were all very scared. Two of the youngest crew members could not stop trembling. They were crying. The older men tried to calm them down, but it is hard to reassure someone when everybody realises that the next bomb might blow up the boat.”
Eventually, at around 10pm, the leading engineer said he could no longer balance the U-boat, and the captain ordered that it should go up to the surface. Then, as the surrounding destroyers fired at them, everyone was told to evacuate.
Last out were the engineer and his assistants. Dethlefs only found out later that they had botched the sinking of the U-boat. They had damaged the levers that would have flooded the ballast tanks by snatching at them before the pins holding them in place were removed. No one had thought to have a bucket of water handy in which the codebooks, whose text was printed in water soluble ink, could have been immersed, thereby causing them to become unreadable.
Thornton barked out the order that the U-boat was to be boarded, and his men, by now “brainwashed” into obeying his every command whatever the risks, complied. There is a dispute over how Tony Fasson, his 29-year-old 1st lieutenant, and 22-year-old Able Seaman Colin Grazier made it to the abandoned U-boat. Romantics say that they, followed by the 16-year-old canteen assistant Tommy Brown, stripped off their clothes and swam over to her. Thornton’s official report states more prosaically that they “jumped over from the bows” as Petard’s bow floated alongside the U-boat’s stern.
What is certain is that all three climbed down into the conning tower to retrieve the codebooks. According to Brown: “The lights were out. The 1st lieutenant had a torch. The water was not very high, but rising gradually. 1st lieutenant was down there with a machine gun which he was using to smash open cabinets in the commanding officer’s cabin. He then tried some keys that were hanging behind the door and opened a drawer, taking out some confidential books which he gave me. I placed them at the bottom of the hatch. After finding more books in cabinets and drawers I took another lot up.”
When he went down again, as he testified: “the water was getting deeper and I told 1st lieutenant that they were all shouting on deck”. Fasson’s response was to hand him more books to take up the conning tower.
The books were placed in one of the destroyer’s rowing boats, just a short distance in front of Dethlefs, who along with a wounded comrade had been rescued from the sea. He remembers thinking: “I am an honourable ‘soldier’. I would do anything to help Germany. I was tempted to reach out and grab the captured papers so I could throw them into the sea. But because I was holding my wounded comrade and because there was a British sailor in front of me holding a gun, I quickly realised that was impossible. I couldn’t do anything.”
Brown’s testimony records what happened on his return to the conning tower: “I shouted, ‘You better come up!’ twice, and they had just started up when the submarine started to sink very quickly.” U559 sank beneath the waves.
Another witness reported: “We yelled and called the names of our shipmates. Only Tommy responded, his head bobbing up almost alongside the sea-boat.”
When he was asked whether there was any possibility of finding the others, he replied: “No chance. They were still down below when I dived off.” It was a tragic end to a heroic action.
But the codebooks recovered from the U-boat, which included the short weather report codebook – used to abbreviate U-boats’ weather reports before the abbreviated version was encoded on the Enigma – were to be instrumental in a dramatic improvement in the codebreakers’ fortunes at Bletchley Park.
Although there was another crisis during the spring of 1943, after the Germans altered the code yet again, it was quickly resolved, this time by using the short signal codebook which had also been retrieved from U-559 by Fasson and Grazier. On 19 March 1943 Churchill was informed by the head of the Secret Intelligence Service. He responded: “Congratulate your splendid hens.” (He liked to refer to the codebreakers as “the hens who laid golden eggs and never cackled”.) From that moment the naval Enigma code used by the U-boats in the Atlantic and Mediterranean was broken more or less every day during the rest of the war.
Sadly Tommy Brown, the sole survivor of the U-559 raid, was destined never to find out what his, and his shipmates’, heroism had achieved. Shortly before the end of the war he failed to extinguish his cigarette when he returned home following a night out drinking while on leave, and the resulting fire killed both him and his four-year-old sister Maureen.
The updated paperback edition of Hugh Sebag-Montefiore’s Enigma: The Battle for the Code is out now, published by Orion’s Weidenfeld & Nicolson. The paperback of his book on the Battle of the Somme will be published on 2 November
