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[Creeperopolis]

The initial campaign of the Breton Civil War took place in 1341 when a French royal army intervened in a dynastic dispute between two claimants to the Duchy of Brittany. Brittany was a province of France, but although the dukes of Brittany were vassals of the French kings they governed the duchy as independent rulers. When Duke John III (r. 1312–1341) died on 30 April 1341, title to the duchy was claimed by both his niece, Joan of Penthièvre, and his younger half-brother, John of Montfort. Joan's claim was exercised through her husband, Charles of Blois, a nephew of the king of France, Philip VI (r. 1328–1350). A complicating factor was the ongoing Hundred Years' War between France and England that had broken out in 1337. A truce was in place which was due to expire in June 1341 but was extended to June 1342.

It was generally assumed that Joan's claim would prevail and that Charles would become the new duke. But John acted quickly and installed friendly garrisons in most of the towns and castles of Brittany by August. He discussed the possibility of a military treaty with English emissaries but made no move towards effecting one. Rumours of these discussions reached Philip, causing him to turn against John and in September Charles was recognised as the new duke. John refused to give way and Philip sent an army to Brittany to impose Charles.

Within a month John had been defeated and was a prisoner. His wife, Joanna of Flanders, sent the ducal treasury west to Brest, took command of her husband's field army, stormed the town of Redon and moved to the small but strongly walled town of Hennebont. She despatched a senior counsellor to encourage English military intervention, set up her two-year-old son, also named John, as the faction's figurehead and heir to his father's claim to the duchy, and waited on events. The war lasted 24 years, frequently as a part of the Hundred Years' War.