A cryptic letter linked to Napoleon Bonaparte has been deciphered for the first time in more than two centuries.
The message was sent in 1809 to French general Auguste de Marmont by Napoleon’s stepson, Eugène, who served as Viceroy of Italy.
After an opening sentence in French, the following 24 lines descend into a jumble of two-digit numbers, isolated letters and unfamiliar symbols.
The original document would likely have been accompanied by a cipher key, but that guide disappeared long ago, leaving the letter’s contents undeciphered.
Carter Church, an engineer with cybersecurity firm SentinelOne, has now solved the puzzle with the help of artificial intelligence.
“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 six hours from a single image and goal,” he wrote on X.
A cryptic letter associated with Napoleon Bonaparte has been decoded after more than 200 years
On March 16, 1809, Napoleon wrote from Paris to Eugène, asking him to forward a letter to Auguste de Marmont, an ally of the Emperor whose forces were stationed in Dalmatia, across the Adriatic.
Because Austrian territory separated Marmont from the other French armies, his troops risked being cut off if war broke out.
The letter begins in French with the sentence: “Vous avez dû recevoir, Monsieur le Général Marmont, mes lettres des 8, 14 et 20 courant.”
The line translates as: “You must have received, General Marmont, my letters of the 8th, 14th and 20th of this month.”
But because the remainder was written in an elaborate code, the message behind it remained unknown.
Church enlisted GPT-6 Astra, an artificial intelligence model developed by OpenAI, to investigate the cipher.
After six hours of analysis, the AI had reconstructed the hidden message.
“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,” Church wrote in a blog post.
The decoded letter maps the positions of French and allied forces and tells Marmont not to be “intimidated” by his isolated location.
“The Russians are marching against Austria, and the army [of] Friuli today numbers eighty thousand men,” the newly deciphered text states.
‘Add the Dalmatian corps, without counting the reserves being organised everywhere in the rear, and you see that Austria will be attacked at once on every side.
‘She is consequently hastening toward her ruin.
‘When you receive the order to march, you must not allow yourself to be intimidated by a few troops or a gathering of rabble.’
To crack it, Mr Church turned to GPT-6 Astra – an AI model created by OpenAI. Over six hours, the bot was able to decipher the message
On 16 March 1809, Napoleon (pictured in painting) wrote from Paris to his stepson, Eugene, asking him to send a letter to Auguste de Marmont – a friend of the Emperor, whose troops were positioned in Dalmatia, on the far side of the Adriatic
According to Mr Church, the cipher itself wasn’t actually difficult to crack.
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However, he admits it would have taken humans much longer than the AI, which was able to whizz through it in just six hours.
He added: ‘The cipher itself isn’t difficult, and cryptographers could have cracked this easily given the time/energy.
‘But, the work to transcribe, catalog, translate, then crack is involved and multi-disciplinary.
‘Every field has an unsolved backlog of niche problems that specialists don’t have the bandwidth to tackle.
‘The takeaway here is that the people shortening those lists will increasingly be outsiders who were simply curious.’
WHAT DO WE KNOW ABOUT THE NAPOLEONIC WARS?
The start of the 19th century was a time of hostility between France and England, marked by a series of wars.
Throughout this period, England feared a French invasion led by Napoleon. Ruth Mather explores the impact of this fear on literature and on everyday life.
Following the brief and uneasy peace formalised in the Treaty of Amiens (1802), Britain resumed war against Napoleonic France in May 1803.
The start of the 19th century was a time of hostility between France and England, marked by a series of wars. Throughout this period, England feared a French invasion led by Napoleon (left). The Duke of Wellington (right) defeated him in battle
The return to war required the resumption of the mass enlistment of the previous ten years, especially as fears of a Napoleonic invasion once again intensified.
The Corsican general Napoleon, soon to become emperor, had made no secret of his intentions of invading Britain, and in 1803 he massed his huge ‘Army of England’ on the shores of Calais, posing a visible threat to southern England.
Hostilities were to continue until the British victory at the battle of Waterloo in 1815.
The Battle of Waterloo was fought on 18 June that year between Napoleon’s French Army and a coalition led by the Duke of Wellington and Marshal Blücher.
The decisive battle of its age, it concluded a war that had raged for 23 years, ended French attempts to dominate Europe, and destroyed Napoleon’s imperial power forever.
The Battle of Waterloo was fought on 18 June that year between Napoleon’s French Army and a coalition led by the Duke of Wellington (pictured on horseback) and Marshal Blücher
The French Emperor Napoleon Bonaparte had escaped from exile in March 1815 and returned to power.
He decided to go on the offensive, hoping to win a quick victory that would tear apart the coalition of European armies formed against him.
Two armies, the Prussians led by Field Marshal Gebhard von Blücher and an Anglo-Allied force under Field Marshal the Duke of Wellington, were gathering in the Netherlands.
Together they outnumbered the French. Napoleon’s best chance of success was therefore to keep them apart and defeat each separately.
Attempting to drive a wedge between his enemies, Napoleon crossed the River Sambre on June 15, entering what is now Belgium.
The next day the main part of his army defeated the Prussians at Ligny and drove them into retreat, with losses of over 20,000 men. French casualties were only half that number.
Pursued by Napoleon’s main force, Wellington fell back towards the village of Waterloo. Unknown to the French, the Prussians, although defeated, were still in good shape.
They retreated northwards towards Wellington’s position and were able keep in contact with him.
Emboldened by their promise of reinforcements, Wellington decided to stand and fight on June 18 until the Prussians could arrive.
The victorious allies entered Paris on July 7. Napoleon surrendered to the British and was exiled to St Helena.