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.

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.

Joan was the only child of John III's dead younger brother Guy, and was married to Charles of Blois, a well-connected and militarily oriented French nobleman who was also a nephew of the King of France.

In early June 1341 John moved on to an alternative plan: to take control of as much of Brittany as he could, and so encourage Philip to confirm him as the new duke to avoid possible conflict.

John left Nantes to secure Champtoceaux, a large ducal castle about 30 kilometres (19 mi) to the north east on the south bank of the River Loire.

John's loyalty at this point was unquestioned and so expelling him by force when he had a strong legal case was politically unappealing, and ran the risk of driving him into an alliance with the English.

Philip passed the question of who should succeed John III to the Parlement of Paris – a judicial rather than legislative body – where deliberations were liable to be long drawn out.

[18] Once the issue was definitively decided in favour of Charles, his nephew, the idea of bringing the traditionally semi-autonomous province more firmly under royal control was attractive to Philip; he was willing to commit considerable military resources, which were available due to the truce with England.

[22] Strategically Edward had the opportunity to set up a ruler in Brittany at least partially under his control, which would provide access to Breton ports, greatly aiding England's naval war and giving ready entry to France for English armies.

[19] In late September the French gathered a 7,000-strong army together with a strong force of siege artillery at Angers, 80 kilometres (50 mi) east of the Brittany border, to enforce Charles's claim.

In early October Charles of Blois led the advance guard of the French army along the Loire valley as far as Champtoceaux and laid siege to it, starting the Breton Civil War.

John was heavily outnumbered but boldly advanced with a small force to a fortified outpost at L'Humeau, 5 kilometres (3 mi) west of Champtoceaux.

The speed of the French advance cut off John from his main field force, which was concentrated around Saint-Renan in the far west of Brittany waiting for English reinforcements.

Less than a week after the French arrived, and after one such attack led to the deaths of many of a large force of townsmen who had accompanied John, a town assembly insisted that he open negotiations for a capitulation.

This was a small but strongly walled town with access to the sea and from there Joanna took control of what forces were still lending their allegiance to her husband's cause, setting up her two-year-old son, also John, as the faction's figurehead and heir to his father's claim to the duchy.

[30][27] When John surrendered he agreed to give up all of his Breton possessions and abide by King Philip's ruling regarding the inheritance of the Duchy of Brittany, and had been promised a safe conduct to Paris and back.

Philip proposed to John that he repudiate all claim to Brittany and his possessions there in favour of Charles of Blois, receiving an annuity and land in France in exchange.

The next year Joan of Penthièvre, Charles' widow, signed the Treaty of Guérande, recognising John's son as Duke of Brittany, which ended the war.