A 24-line cipher from a letter sent by Napoleon's son-in-law, Eugene de Beauharnais, to Marshal Auguste de Marmont in March 1809 has been decoded by the AI model GPT-6 Astra, developed by OpenAI. The letter had remained undeciphered for 217 years after being published in a French army journal in 1969 and later digitized by Persée, a digital library. The cipher used 155 distinct characters out of a total of 1,300 symbols, employing a homophonic code in which common letters had multiple interchangeable symbols. This method made the message more difficult to break without knowing the key. Carter Church, an AI engineer at SentinelOne, used GPT-6 Astra to process a low-resolution scan of the letter. The AI analyzed the image line by line, transcribed 1,300 units, and reduced the symbols to 155 distinct characters. It then identified a partial table from cryptology historian Daniel Tant, which documented 33 symbols used in other exchanges between Eugene and Marmont. Using this information, the AI employed simulated annealing, a computational method for problem-solving, and was fed with French texts from authors such as Victor Hugo and Alexandre Dumas, as well as the memoirs of Marmont, to decode the remaining symbols. The decoded letter informs Marmont of the positions of the French and allied armies on the continent, including the deployment of 40,000 Bavarians under LeFebvre, 80,000 French troops with Davout at Bayreuth, and 60,000 men for Masséna. It also situates the Austrians at several locations two weeks before their offensive on April 10. The letter contains a state lie about the Russians marching against Austria, a claim that was inaccurate as the Tsar's army had not yet crossed the Galician border. The text also mentions Poniatowski commanding 30,000 Poles, though sources indicate fewer troops were actually present. The letter instructs Marmont to "brave 'some troops or a gathering of scoundrels,'" a paraphrase of a phrase from Eugene. A ghost word remains, as the 1865 edition of the letter interrupts Napoleon's phrase after "a handful of," followed by dots. Five signs remain uncertain, according to Church. Michael Rowe, a historian at King's College in London, notes that while the letter repeats an imperial order already known, its value lies in the method used to decode it, which has restored an expected content and gains credibility for ciphers with unknown content. The decoded message has been ranked among solved ciphers by Satoshi Tomokiyo, who maintains the Cryptiana database. Researchers are excited about the speed and reliability of the technology, with some comparing it to a "Rosetta Stone" for deciphering historical coded messages. The objective now is to use this method to translate numerous secret correspondences from diplomatic and military exchanges, as well as private communications, potentially revealing new insights into historical events and figures.