I know that the kickoff of the 100 Years War was the French king formally confiscating the last county of the Plantagenets, Gascony, from the English crown. Would there be any lasting stereotypes about people from Gascony in Dumas's day, that a contemporary reader would pick up on? Or was there some more esoteric reason for the birth place of the main character?
Historian Alain Decaux, in a lecture to the Académie Française in 1980, told the story of the origin of Dumas' Three Musketeers as follows.
In the late 1830s, Alexandre Dumas was already a celebrated author, but he was known only as a playwright. He started thinking about writing historical novels in the style of Walter Scott, Victor Hugo, and, more recently, Eugène Sue. This kind of novels, now published in serialized form in the press, were highly successful. He then met Auguste Maquet, a professor of history, who brought him the outline of a story, and Dumas turned it into his first novel, Le Chevalier d’Harmental (1841), now mostly forgotten.
Dumas then looked for a new story, and while in Marseilles, he met the head librarian of that city (the brother of a poet friend of Dumas), who lent him the Mémoires de M. d’Artagnan, the alleged memoirs of Charles de Batz de Castelmore, known as d'Artagnan, a Gascony-born officer who served Louis XIV as captain of the Musketeers of the Guard. Dumas forgot to return the book, and with the help of Maquet, who did the first draft, he wrote the Three Musketeers.
Why Gascony? Simply put, for several centuries, this (not well-defined) region had the reputation of a land of brave and valiant soldiers. Florimond de Raemond, in his preface to the Memoirs (Commentaires) of Gascony-born marshal of France Blaise de Montluc, writes in 1592:
Sirs, as we can see in certain countries, that produce abundance of fruit that rarely come from other lands, it seems that your Gascony gives birth regularly to infinite numbers of great and brave captains, as if they were its own and proper fruit.
Raemond then gives a long list of generals and officers from Gascony. Many officers from South-West France were in the Musketeers company, funded by Henri IV (formerly Henri de Navarre, born in Pau), including many members of the Batz/Montesquiou/d'Artagnan family, as well as the models for Athos (Athos d'Autevielle), Porthos (Portheau) and Aramis (Aramitz), and their relatives.
To be fair, Gascogne had also a less favourable reputation. Furetière's dictionary (1701) gives for Gascon the definition of "Braggard, quarrelsome", which also fits (some of) the characters of the novels. It also turned out that the Mémoires of M. d'Artagan were largely fictional and had been written by one of his comrades in arms.
So, in a nutshell, Gascony, a semi-legendary land of bragging, quarrelsome and larger-than-life soldiers, provided the perfect inspiration for Dumas and Maquet, who were trying the make a best-selling historical novel (which does not take place in Gascony). Amusingly, author Cyrano de Bergerac had no relation with Gascony (he was from the Paris region), but this did not prevent Edmond Rostand, in his eponymous play (1897) to turn this libertine writer into an archetypical Gascon, with "gasconne" qualities of brashness, bravery, and blustering.
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