Initial campaign of the Breton Civil War

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 died childless 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. 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.

https://en.wikipedia.org/wiki/Initial_campaign_of_the_Breton_Civil_War