AI deciphers 217-year-old coded letter sent by Napoleon's stepson in 1809 A researcher used GPT-6 Astra to crack a coded letter from Eugene de Beauharnais to General Auguste de Marmont, revealing military positions and Napoleon's mindset before the 1809 Austrian campaign. A coded letter written by Napoleon Bonaparte's stepson, Eugene de Beauharnais, in March 1809 has been deciphered after 217 years using artificial intelligence. The letter was sent on Napoleon's request by Eugene, the Viceroy of Italy, to French general Auguste de Marmont, who was stationed with his troops in Dalmatia on the eastern shore of the Adriatic Sea. Marmont was a close companion of the emperor and was totally isolated from other French forces and their allies. The breakthrough was made by Carter Church, who used GPT-6 Astra to break the unsolved cipher. Church said he used GPT-6 Astra to break an unsolved cipher to one of Napoleon's generals that had gone unread for 217 years.AI Deciphers Napoleon's Coded LetterHe noted that what makes this impressive isn't the codebreaking, but that Astra completed the entire multi-modal workflow in about six hours from a single image and goal. The letter's purpose was to tell Marmont the position of every other French and allied army across the continent. To avoid revealing military secrets to France's enemies should the letter be intercepted, the message was written in code. The opening line of Napoleon's correspondence is in his native French.The first line reads: Vous avez du recevoir, Monsieur le General Marmont, mes lettres des 8, 14 et 20 courant, which translates to: You must have received, General Marmont, my letters of the 8th, 14th, and 20th of this month. The next 24 lines are an incomprehensible mix of symbols, letters, and numbers. Experts believe the original correspondence would have come with a deciphering key, but that key has been lost to time, until now.The cipher was published in a French army journal in 1969. It contains 155 different signs, 1,300 signs in total, and the code is homophonic, meaning common letters have several interchangeable symbols. A low resolution scan and a single prompt was all GPT-6 Astra needed to transcribe the text and find a historian Daniel Tant, previously unlinked to this letter, who documented what 33 of the symbols meant, accounting for about a third of the letter.Church said Astra made this connection itself via web research as it was investigating. He explained that Tant catalogued the table as Code entre Eugene de Beauharnais et le General Grammont, which may have been a naming mistake. Either way, Astra identified this example and decided to try and apply it to this cipher, and it worked.How Astra Broke the CipherAccording to Church, Astra cut the 1,202 by 1,836 plate into rows, read the signs, decided which of 175 hand-drawn marks were the same sign written twice, and only then began breaking the cipher. GPT-6 successfully used the partial table to determine what each of the other symbols stood for and translate the message. Church said he did not believe it at first.Everyone has the experience of seeing a model claim something outrageous, but the difference here is that the solution is mechanically testable: there is essentially zero chance an incorrect key could produce coherent and historically consistent output. He also said the verifiability of solutions is actually the most difficult part, far more difficult than generating potential solutions. You simply cannot automate accountability.As the AI deciphering revealed, the letter outlined the locations of every other French and allied army, urged Marmont not to lose hope or be intimidated by his isolated position, and assured him that Austria was speeding toward doom. The translated message states: Add the Dalmatian corps, without counting the reserves being organized everywhere in the rear, and you see that Austria will be attacked at once on every side. She is consequently hastening toward her ruin.It also says: When you receive the order to march, you must not allow yourself to be intimidated by a few troops or a gathering of rabble. Eugene added: I continue to await news from you with impatience. Historians have responded to the findings.Historians Assess the TranslationA researcher in European history at King's College London said the findings were impressive and that because we have access to Napoleon's original correspondence to his stepson, where he detailed what the ciphered message should include, we can be fairly confident in the translation of Eugene's letter. The researcher compared it to a kind of Rosetta Stone.If you have tested out AI on this letter, where you pretty much know what the contents would have been because it is referred to in another letter, the AI does its thing and essentially reproduces what you are expecting to see. Then if you use the exact same kind of tool and methodology on something where you do not know, you are more likely to trust the outcome.However, the translation is not too surprising to historians because the cipher version just repeated a large segment of what was in that letter. In a sense it does not tell us anything new, it is not like it reveals some kind of great new information. According to White, the translation does raise some curious questions about Napoleon's mindset in the run up to the campaign in Austria in 1809.For example, Napoleon actually had 60,000 fewer troops available to fight at the time than is claimed in this letter, and he also seems fixated on the number of troops Austria might send to invade Italy. White said it is important that historians now set the AI's work in a bigger picture of the history of the period, adding it will also be interesting to see if the key that has resulted from this AI's work can now be used on other correspondence.Ciphers in Napoleon's Military CorrespondenceCiphers were fairly common during this period, conveying sensitive military and diplomatic information. French Emperor Napoleon Bonaparte was forced to use ciphers as a way of communicating sensitive information securely across his vast empire. It was always a very real possibility that these letters would be intercepted, potentially providing vital intelligence to France's enemies about the Emperor's plans.However, using AI to crack these codes could also offer insights into the private lives of historical figures. Ciphers were often also used for private correspondence, for example letters between elite individuals having extramarital affairs. The technique could also be used by ancient historians studying documents. Church maintains that cracking the cipher was not particularly challenging, but the task would have taken human minds much longer than the six hours AI required.He said the cipher itself is not difficult, and cryptographers could have cracked this easily given the time and energy, but the work to transcribe, catalog, translate, then crack is involved and multi-disciplinary. He noted that every field contains a litany of problems that specialists do not have the capacity to tackle.The Campaign That FollowedJust two months after the letter was sent, on May 21, 1809, during the Battle of Aspern-Essling near Vienna, Austria, Napoleon suffered his first personal defeat, which resulted in the death of 23,000 French soldiers. In the wake of that loss, Napoleon regrouped his forces and defeated Archduke Charles in July of 1809 at the Battle of Wagram. That battle led to the dissolution of the Fifth Coalition, the Austrian and British-led alliance against France.Auguste Frederic Louis Viesse de Marmont was the intended recipient of the coded message while stationed on the eastern shore of the Adriatic Sea.