From Anatole France’s At the Sign of The Reine Pédauque (La Rôtisserie de la reine Pédauque).
“Well then,” M. d’Anquetil continued, “whatever may be printed of it in the gazettes, war consists, above all things, of stealing the pigs and chickens of peasants. Soldiers in the fields have no other occupation.”
“You are right,” said M. Coignard, “and in days of yore it was the saying in Gaul that the soldier’s best friend was Madame Marauding.
—Eh bien! reprit M. d’Anquetil, quoi que disent les gazettes, la guerre consiste uniquement à voler des poules et des cochons aux vilains. Les soldats en campagne ne sont occupés que de ce soin.
—Vous avez bien raison, dit mon bon maître, et l’on disait jadis en Gaule que la bonne amie du soldat était madame la Picorée.
As best I can trace it, the Madame Marauding/madame la Picorée witticism makes its first appearance in the 16th century memoirs of François de la Noue which cover the early wars of religion. In his description of the 1562 fall of Boisgency (modern Beaugency, I think) and the cruelty of the soldiers towards the inhabitants he says:
…So our infantry lost its virginity and from this illegitimate conjunction followed the conception of Madamoiselle La Picorée, who has since so increased in dignity that we now call here Madame. And if the civil war continues on, I have no doubt that she will become a Princess.
The passage and full text are here on google books