The Breton Dumpster Fire: How The War of Breton Succession Blew Up The Hundred Years' War.

If you thought the succession of the French crown was contentious, just wait until you hear about Brittany.

I once heard that the Hundred Years’ War broke out because the French court mocked the English king’s accent so ruthlessly that it prompted him to declare war. Vanity, however, was not the cause of the Hundred Years’ War, nor was the other often cited trope: the succession to the French crown. The truth is actually far stranger.

You see, on the eve of the outbreak of war in 1337, Edward III had not yet claimed the French crown; nor would he do so for another three years. He even paid homage to the new French king, Phillip VI, in 1329, putting the matter to proverbial bed. This tells us that contesting the French crown was not the original motivating factor behind his first campaign in France (though it did later become a matter of political expediency). Instead, modern historians have come to agree that the war had its roots in French aggression.

England and France had fought several small-scale wars in the preceding decades, many of which involved England’s claim to the region of Aquitaine through their Angevin line. King Phillip VI decided to settle this long-standing matter, which was a frequent source of costly conflicts, by simply annexing it in 1337. Edward was none too pleased.

Now, there was a succession controversy that rocked France in 1328, and it needs to be mentioned. Edward did have a claim to the French throne on his mother’s side, and in English law, that would have given him a fighting chance to become king. However, French law did not permit kings to come from the female line, so Edward was denied the right to petition. Given that Edward was sixteen or so when this all happened, it’s hard to say whether he truly understood what was at stake, and he did not press the issue. He did, however, keep the resentment in his back pocket for later.

It could be said that part of Phillip’s motivation to annex Aquitaine stemmed from the earlier succession struggle. Some historians have proposed he was peeved that Edward even tried to contest him. In truth, his reasons were more economic, and I discuss those factors further in my courses.

The first years of the war followed the same unremarkable cadence of the wars of the previous eighty years. A few sea battles, clashes in Flanders and Aquitaine, and a meandering campaign in northern France did little to impress. What's more, wars at the time were expensive and slow, and for monarchs whose revenue systems were still shaky at best (France was even considered to have been in an economic depression), even these small, unimpressive engagements took their toll. By 1340, Edward and Phillip signed their first truce to allow time to refill their war chests.

Had nothing else happened, the war might have ended there. Each monarch might have gone his separate way, and historians might have chalked up the conflict as another one for the books. But it would not be so. No, a small, irreducible region in Western France was sitting on a time bomb that would fan the flames of war for the next 123 years. Brittany, still a semi-independent duchy that played both sides in the conflict between France and England, experienced its own succession struggle. The trouble was, despite their semi-independence, their nobility was intermarried with half the continent.

When Duke John III of Brittany died on 30 April 1341, he had no clear successor. Two claimants emerged: Charles of Blois and John of Montfort. Charles’ claim stemmed from his marriage to Joan of Penthièvre, the niece of Duke John III and the daughter of his younger brother, Guy. John’s claim stemmed from his being Duke John III’s half-brother, making his a male-line claim.

Now, Charles had a slight advantage insofar as he was also Phillip VI’s nephew. Naturally, Phillip supported his claim. But herein lay the incendiary problem: Edward had been denied a right to claim the throne of France because Capetian Salic Law did not allow succession through the female line. But Phillip was now pressing his advantage in Brittany despite Charles having a claim through a woman. Edward, of course, took great offense to Phillip’s hypocritical move, and that festering resentment over the female succession controversy took hold. He thus supported John, not just out of principle, but also because there were a lot of damn sheep at stake.

Forgive my expletive, but sheep were really important. How important? They were so immensely, tremendously, stupendously valuable that any shift in who controlled them stood to become the next hegemon of Western Europe. And they powered England’s war machine. Wool from England was sent to the Low Countries to be spooled, refined, and dyed, and then sent to France to feed its textile industry, which sold into the rest of the world. At the time, the textile industry was THE defining industry of the European economy, akin to today’s giant waste-of-money gamble on AI in the U.S. economy.

Therefore, whoever controlled Brittany stood to make immense gains. And they might use those gains for…you know…more wars. You can see, then, that the stakes were ridiculously high. Brittany had other valuable assets, such as strategic ports, salt, and other commodities. But since Edward had taken out loans against his wool industry to fund his war efforts, he stood to gain the most by accessing the Breton wool market.

The War of Breton Succession thus poured rocket fuel on an already smoldering fire, leading to the renewed outbreak of hostilities between England and France that would persist long after everyone involved was dead. Still, it’s neat to think that the small region of Brittany, which the Romans had considered a waste of space (the peninsula was depopulated during the Gallo-Roman period), was in large part responsible for turning the broader conflict we today call the Hundred Years’ War into something more than a mere continuation of the sporadic disputes of the previous century.

Want to learn more? I’ll be teaching an in-depth course on the Hundred Years’ War in 2026. Enroll below: