When A Mesnier wrote to his mother in December 1870, Paris was under siege. The Prussians had surrounded the capital, food supplies were running low and temperatures had dropped below freezing, reports BBC News.
Mr Mesnier wrote of his hopes for victory and his frustration at being unable to enlist. The letter was flown out of Paris in a "ballon monté", a hot air balloon, the only way to communicate with the rest of France.
Recently the letter has inexplicably resurfaced in Australia. But how it got there remained the big question.
When Emmanuel Hamel, a civil servant in Normandy, read about the letter in a local newspaper, he immediately set out on his investigation.
The first clue, he said, was the mother's name Mesnier and the address in Pont-Audemer, Normandy. Mr Hamel calculated Mrs Mesnier's likely date of death and found her death certificate in local archives. Her full name was Julie Stephanie Mesnier.
But it was the next discovery that gave Mr Hamel a frisson.
Trawling through archives, he stumbled upon the marriage certificate of Mrs Mesnier's son Pierre Alexandre with a woman in Paris.
"I started getting palpitations, I knew I had found him," he said. "The signature on the marriage certificate matched the signature on the letter!"
So A Mesnier, the author of the letter, was in fact Pierre Alexandre Mesnier, a lawyer at the court of Napoleon the Third.
"It is wonderful to have a sense of substance, to know that Mr Mesnier was working at the imperial court, hence the fervour and engagement with Paris," says Louise Doyle, assistant director general at Australia's National Archives, who has been in touch with Mr Hamel about his discoveries.
But there was more to come.
"In the letter, the author mentioned a man called Jules who had not written to him for a long time," Mr Hamel said.
"I assumed Jules was a close relative because there was no surname." And indeed the name Jules Mesnier also appears on Alexandre's wedding certificate as his witness and brother.
Jules Mesnier managed a Swansea-based shipping company called Poingdestre et Mesnier. He also founded France's first phosphate mining company, Phosphate de l'Oceanie, which traded with islands across the Pacific Ocean.
Quite conceivably his job would have taken him to Australia and the Pacific.