She decoded ENIGMA – How a 19-Year-Old Girl’s Missing Letter Killed 2,303 Italian Sailors

March 27, 1941, and the Mediterranean Sea lay still under a moonless sky. The Italian fleet, led by Vice Admiral Angelo Yachino aboard the battleship Vittorio Veneto, prepared for a decisive strike. His fleet, a formidable force of four destroyers, three heavy cruisers, and the flagship itself, was poised to obliterate any British resistance in the area. Armed with the most powerful naval force Italy had assembled since the war began, Yachino believed he was on the cusp of victory. But he had no idea that 1,200 miles away, in a dimly lit mansion outside London, a 19-year-old woman was about to notice something no one else had—something that would set into motion a series of events that would doom his entire fleet.

Mavis Batty, a young woman without formal training in mathematics or cryptography, was working at Bletchley Park, Britain’s top-secret codebreaking facility. Her job, alongside a team of highly trained cryptanalysts, was to decipher Italian naval communications encrypted using the Enigma machine—an unbreakable cipher that had baffled the British for months. These machines, gifted to Mussolini by Hitler himself, were the epitome of wartime encryption. Every day, the Italians changed the machine’s settings, making it mathematically impossible to crack the code in any reasonable amount of time. Or so they thought.

For 18 months, Mavis and her colleagues had been staring at what appeared to be random gibberish. Hours spent sifting through the mess yielded nothing, and most of the codemakers—many with advanced degrees in mathematics—were ready to give up. But Mavis couldn’t shake the feeling that something was wrong. She wasn’t a cryptographer. She didn’t have the mathematical expertise of her colleagues, but she had something they didn’t: an extraordinary ability to spot patterns where others saw only chaos.

On March 25th, 1941, Mavis was working the night shift when she noticed something peculiar in an encrypted message. Among the seemingly random jumble of letters was a single word, “per x,” repeated three times in different positions. This was a critical break. In the world of encryption, repeated words were often the key to cracking the cipher. Cryptanalysts had long known that if they could identify a repeated word, they could reverse engineer the encryption settings.

The problem was, Mavis had no idea what “per x” meant. She ran through possibilities in her mind—perhaps it was a ship name, coordinates, or a weather report. But nothing fit. Then, an idea hit her. What if “per x” wasn’t a real word at all? What if it was filler text, used by the Italian operators out of habit? She remembered German communications, where operators sometimes added random filler words when they weren’t sure what to write, just to meet the required message length.

Could “per x” be a placeholder, a word that didn’t mean anything but was used simply to fill the space? She rushed to her supervisor, Dilly Knox, who had worked for British Intelligence during World War I and was a renowned cryptanalyst. Knox was legendary for his work in breaking German codes, and Mavis had always admired him. But even Knox was stumped by the Italian naval Enigma.

When Mavis woke him from his sleep, he studied the message for just a few seconds before his eyes widened in realization. The term “per x” was, in fact, filler—padding. And if that was the case, it meant the Italians had made a mistake. This simple mistake could allow them to crack the code, and possibly even prevent a disaster.

Mavis and Knox immediately assembled a team of cryptanalysts, mathematicians, and experts to reverse engineer the encryption. They had 72 hours before the Italians would change their Enigma settings, effectively closing the window of opportunity forever. But the mathematics were brutal. Even with a single word to work from, they had to manually test thousands of possible combinations to identify the right settings.

Mavis worked tirelessly, her method more intuitive than mathematical. While the others relied on mechanical calculators and number crunching, Mavis tested combinations by hand, feeling out the settings based on patterns she had noticed in previous messages. After 37 hours of non-stop work, she had a breakthrough: “per x” appeared again in another message, encrypted slightly differently. By comparing these two, she was able to identify the plugboard setting for the letter “P.” It was a small step, but it was a crack in the dam.

Over the next few hours, Mavis and her team worked feverishly to test the Enigma settings. The pressure was immense, but Mavis refused to sleep. Her dedication was unwavering. At hour 63, they had identified the wheel order. By hour 68, they had cracked the ring positions. With only 4 hours left, Mavis and her colleagues completed the plugboard configuration.

The Italian Enigma code was broken.

The first message they decrypted was a routine weather report. The second one, however, was far more important. It was an operational order from Admiral Yachino, outlining the movements of his fleet—destroyers, heavy cruisers, and battleships—heading out from Italian ports to intercept British convoys. The coordinates were precise. The British had the Italians’ exact plan, and they knew exactly where the fleet would be.

Mavis Batty had just given the British navy an enormous advantage.

Admiral Andrew Cunningham, the commander of Britain’s Mediterranean fleet, now faced a difficult decision. The Italian fleet, especially the battleship Vittorio Veneto, outgunned anything he had. But thanks to Mavis’s breakthrough, Cunningham knew exactly where the Italians would be and when. The intelligence was so accurate, it seemed too good to be true. He ordered his fleet into action, under strict radio silence to ensure that the Italians would think they were still unaware of the threat.

The Italians sailed straight into the trap. For two hours, they pursued what they thought was a retreating British fleet. But by nightfall, they found themselves in the sights of British battleships, waiting to ambush them. The ensuing battle, known as the Battle of Cape Matapan, was devastating for the Italians. In just 247 minutes, British ships destroyed three heavy cruisers, two destroyers, and badly damaged the Vittorio Veneto. Over 2,300 Italian sailors died.

The British casualties were minimal—three men killed, and one aircraft lost.

The Italian fleet was crippled, and the British navy had Mavis Batty to thank for it. The young woman who had no formal training in cryptography had just helped turn the tide of the war in the Mediterranean. She had cracked a code that had been deemed mathematically impossible to break and had given the British forces the intelligence they needed to defeat a superior enemy fleet.

But Mavis never sought recognition for her work. She continued her life quietly after the war, raising a family and pursuing a career in garden history. It wasn’t until the British government declassified Bletchley Park’s wartime activities in the 1970s that her role in the victory at Cape Matapan came to light. Even then, Mavis played down her accomplishments, insisting that she had just done her job. But the truth was undeniable: her breakthrough had saved thousands of lives and played a crucial role in one of the most lopsided naval battles in modern history.

Mavis Batty’s success was a testament to the power of intuition, observation, and dedication. A 19-year-old woman, with no formal training in mathematics, had cracked an encryption system that defeated experts with advanced degrees. She had done what many thought was impossible, and in doing so, had changed the course of the war.

And in the end, it wasn’t just her intellect that won the day. It was her ability to notice something that everyone else had missed—a single missing letter that, when decoded, would save thousands of lives and help secure victory in the Mediterranean.