As soon as the travelling crate was opened and the shroud of white tissue paper carefully peeled away, it was clear there was damage to the dark blue coat: a hole in the left shoulder, and some of the gold braid on the epaulette torn away. The damage happened more than two centuries ago, and the coat's arrival in France was one of the most unusual days in the history of the National Maritime Museum, in Greenwich, south London, and the Musée de l'Armée, at Les Invalides in Paris.

"I think it's a wonder," said Emelie Robbe, a curator of the Paris museum's new exhibition on Napoleon and Europe. "It is astonishing that it should be here."

The coat, an undress uniform of the Royal Navy, already slightly old fashioned when it was made in the late 18th century, had never left England since 1805, when it came back in a sea chest on the same ship that carried the body of Horatio Nelson preserved in a barrel of brandy. It has now voyaged again, through the Channel tunnel, into the heart of his enemy's empire....