A secret military message sent during Napoleon Bonaparte’s 1809 campaign has been deciphered more than two centuries after its coded text resisted modern researchers. The breakthrough used AI to reconstruct the Napoleon-era cipher, revealing a briefing prepared for Gen. Auguste de Marmont as France and Austria moved toward war.
The message was not written by Napoleon himself. It came from the headquarters of his stepson, Eugène de Beauharnais, the viceroy of Italy, and was sent to Marmont, whose French force was isolated in Dalmatia. On March 16, 1809, Napoleon ordered Eugène to send Marmont a coded account of French and allied troop positions.
AI engineer Carter Church, who works at cybersecurity company SentinelOne, published the decipherment Sept. 18. Church said OpenAI’s GPT-6 Astra worked from a low-resolution reproduction printed in a French military journal in 1969 and spent about six hours on the task. He later checked the result against the image and surviving historical documents.
AI helps solve a military Napoleon-era cipher from his 1809 campaign
The cipher contains 1,300 units and 155 distinct signs. It used a homophonic system, meaning common letters could be represented by several different symbols. A partial table compiled earlier by cryptology historian Daniel Tant identified 33 letter values, accounting for about one-third of the message.
I used GPT-6 Astra to break an unsolved cipher to one of Napoleon's generals that had gone unread for 217 years.
What makes this impressive isn't actually the codebreaking, but that Astra completed the entire multi-modal workflow in ~6 hours from a single image and goal.
1/6🧵 pic.twitter.com/vyiGKOsrHq
— Carter Church (@CarterWChurch) September 30, 2026
The decoded briefing lists French and allied formations across Europe, including Bavarian forces under Marshal Lefebvre, Polish troops under Prince Józef Poniatowski, and French forces commanded by Davout, Masséna, and Oudinot. It also describes Austrian positions around Laibach, Klagenfurt, Villach and Salzburg.
A major check on the reading comes from Napoleon’s surviving correspondence. His March 16 order told Eugène to inform Marmont, in cipher, of the positions of the same armies that appear in the reconstructed message. The dispatch also opens by referring to earlier letters dated March 8, 14, and 20, which survive in Marmont’s published memoirs.
The decoded message fits the crisis before Austria’s attack
One passage tells Marmont not to be intimidated by “a few troops or a gathering of rabble” when ordered to move. The letter also says Russian forces were marching against Austria, although other correspondence cited in Church’s research indicates Napoleon was still pressing for Russian action at the time.
Austria opened the war in April 1809. Marmont later fought his way north from Dalmatia, reached Laibach in June and joined Napoleon’s army for the Battle of Wagram in July.
Church said five cipher signs remain uncertain, but the alternatives do not change the message’s overall meaning. He also released the transcription, reconstructed key, and verification scripts, allowing other researchers to test the proposed solution.
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