The boarding of U-559 changed the war – now both sides tell their story

In an updated extract from the bestselling Enigma history, we revisit the daring engagement in which two men gave their lives to make a breakthrough possible

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 archaeology best 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.