AI Deciphers 217-Year-Old Coded Military Letter Ordered By Napoleon

The letter was not written by Napoleon himself. Napoleon ordered his stepson, Eugène de Beauharnais, then Viceroy of Italy, to send the coded message to General Auguste de Marmont, whose French forces were stationed in Dalmatia, in present-day Croatia.

Carter Church, an artificial intelligence engineer at cybersecurity company SentinelOne, said he used OpenAI's GPT-6 Astra model to transcribe and decipher the document from a single scanned image. The process took about six hours.

The message had remained unsolved because its original cipher key was lost. The surviving reproduction contains more than 1,000 encrypted units made up of numbers, letters and hand-drawn symbols. Church's research identified a complex homophonic cipher containing about 155 distinct signs, making conventional frequency-based analysis difficult.

The decoded text provides a military briefing on the positions and strength of French and allied forces across Europe. It refers to French troops in Bavaria, Saxony, Poland and northern Italy, as well as the movements of Russian forces and the strength of the army in Friuli. The message was also intended to reassure Marmont that French and allied forces were strong enough to confront Austria.

According to Church's account, the AI did more than simply read the scanned characters. It analyzed recurring symbols, reconstructed likely relationships between the cipher signs and French letters and words, and generated a complete reading that could then be tested against historical records. A partial cipher table prepared by French cryptology historian Daniel Tant provided additional evidence for the reconstruction.

The decoded message also provides new context for Napoleon's military planning. A surviving letter from Napoleon to Eugène dated March 16, 1809, had already instructed him to communicate the positions of French and allied armies to Marmont in cipher. The newly decoded document closely matches those instructions, providing an important historical cross-check of the AI-assisted reading.

The research also revealed that some of the intelligence in the letter was inaccurate or deliberately optimistic. For example, the message described Russian forces as marching against Austria, although Napoleon himself later expressed uncertainty about whether Russia would actually enter the conflict. The letter therefore appears to have served not only as a military briefing but also as an effort to bolster confidence among an isolated French commander.

The document was reproduced in a 1969 article in the French military-history journal Revue historique des Armées. Until the recent breakthrough, researchers had access to the reproduction but not the original cipher key needed to read the encrypted section.

The discovery highlights a growing application of artificial intelligence in historical research. In this case, the technology combined image analysis, pattern recognition and historical cross-checking to reconstruct a cipher that had resisted human researchers for generations, while historical expertise remained essential for testing and interpreting the result.

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