IN 1806 Josephine Bonaparte wrote to one of Napoleon’s most-trusted generals, Alexandre Berthier, who had also become her own close friend, appealing to him to guard the emperor’s safety in battle. She could not have known it, but it was her future happiness which was in danger.

“Above all take care of the emperor,” she wrote in October 1806 to his chief of staff. “Ensure that he does not expose himself too much, you are one of his oldest friends and it is your attachment to him which calms me.”

A few weeks after she wrote so tenderly of Napoleon’s welfare, his mistress Eleonore Denuelle would give birth to a son — convincing him that it was his wife’s fault, not his, that they had failed to produce the longed-for heir. The fate of their marriage was sealed.

The letter is one of four revealing unpublished letters from Josephine to Berthier — held within his family for more than two centuries — to be auctioned by Sotheby’s in Paris on Tuesday (today).

The sale also includes orders from Napoleon to Berthier to inspect the defences of ports, towns and camps along the English Channel, before the men visit them together in the following year in preparation for his planned invasion of England.

Josephine de Beauharnais married the considerably younger Napoleon in 1796, to the horror of his family, when he was a young army officer and she was a widow with two small children supported by a string of lovers. She partly befriended Berthier in sympathy for his own irregular household: he was devoted to his married Italian mistress, and she welcomed both of them to her home.

By the time of the letters Josephine was empress, crowned by Napoleon in a dazzling ceremony in Paris in 1804. All her life she was infamous for extravagance, and one of the letters is on notepaper of truly imperial luxury, bordered with embossed palm trees, classical urns and laurels, and decorated with now tarnished silver leaf.

“These are wonderful letters, so intimate and revealing, to a man who was a trusted friend,” Frederique Parent, an expert at the Paris branch of the auction house, said. “It has been a privilege to have them in my hands.”

In the last of the letters, in 1809, she wrote to Berthier of the death of Jean Lannes, one of Napoleon’s most brilliant generals who also became a close friend of her’s. Napoleon said of him: “I found him a pygmy and left him a giant.”

Parent described the letter as “exceptionally unusual”, adding: “To find Josephine directly addressing military affairs, and a general by name, is quite remarkable.”

Lannes was badly injured in the French defeat at the battle of Aspern-Essling in 1809, when the Austrians drove Napoleon back from an attempted crossing of the Rhine. Both Lannes’s legs were amputated. He died eight days later of fever.

Josephine appeared worried that she had given false hope of his recovery to his widow, writing: “Everything that I had learned made me believe the Marechal out of danger.” Concerned as ever for Napoleon, and worried about her son Eugene, she ended the letter: “Be assured of my tender friendship, embrace Eugene for me, I am happy to know him close to the emperor and to you.”

Later that year her marriage to Napoleon would be tested to breaking point. He finally decided to be rid of her, in his determination to obtain a male heir. Her screams when he broke the news are said to have roused the entire household. He asked Berthier for evidence of her infidelity to help with the divorce, but was refused. He later sent his friend to open negotiations for marriage to the daughter of Francis II, the emperor of Austria. In 1810 the divorce was finalised and he married Marie Louise, duchess — and the following year they did had a son, Napoleon II.

The sale on Tuesday, predicted to raise several million euros, also includes uniforms and swords, family portraits and records, books and documents spanning Berthier’s career.

Josephine lived on until 1814, when she died of pneumonia after contracting a cold while walking in her famous rose gardens at Malmaison with Tsar Alexander. Napoleon died in exile on the remote island of St Helena in 1821: his last words were recorded as “France, armee, tete d’armee, Josephine” (France, army, head of the army, Josephine).

—By arrangement with the Guardian