[2] The war formed an integral part of the early Hundred Years' War due to the proxy involvement of the French and English monarchs in the conflict; the French supported the Blois (female heir) whilst the English backed the Montforts (male heir).

Since John III was childless, his heir of choice became Joan of Penthièvre, la Boiteuse, daughter of his younger brother Guy.

In 1337 she married Charles of Blois, the second son of a powerful French noble house whose mother's brother was King Philip VI of France.

Most of the nobility supported Charles of Blois, so if John of Montfort was to have any chance, it was dependent upon swift action before organized resistance could be made.

Thus, when rumours reached Philip VI of France that John of Montfort had received English agents, the French Crown naturally took a more direct interest in their small neighbors situation.

In November, after a short siege and defeat at the Battle of Champtoceaux, John of Montfort was forced to surrender at Nantes by the citizens.

The siege dragged on and a French army was assembled to meet him, but on 19 January 1343, before any major engagements could be fought, the two kings agreed the Truce of Malestroit.

With John of Montfort in prison, his son an infant, and his wife recently gone mad, the places under Montfortist control were in practice administered from London, with a large permanent English garrison at Brest.

In Paris, John of Montfort was released from prison on 1 September 1343 in return for a huge bond and a promise to stay on his estates in the east.

They had some successes, such as the expulsion of the papal custodians from Vannes, but with no unifying leadership, mostly they were reduced to pleading for men and money from London.

As part of his larger strategy, a force was dispatched to Brittany under the joint leadership of the Earl of Northampton and John of Montfort.

Within a week of their landing in June, the English had their first victory when Sir Thomas Dagworth, one of Northampton's lieutenants, raided central Brittany and defeated Charles of Blois at Cadoret near Josselin.

During the winter, Northampton fought a long and hard campaign with the apparent objective of seizing a harbour on the north side of the peninsula.

It was during a tour through the English strongholds on 9 June that Dagworth and his escort were trapped by Charles of Blois and his army near Saint-Pol-de-Léon.

The memory of the massacre at Quimper increased his unpopularity, and Breton traders had an economic interest in strengthening links with England due to Brittany's strategic position between the Atlantic and English Channel.

Under pressure from Pope Innocent VI, the English, French, and Bretons negotiated a peace, while both factions maintained an uneasy balance of power within the duchy.

Edward III signed the Treaty of Westminster on 1 March 1353, accepting Charles of Blois as Duke of Brittany if the latter undertook to pay a ransom of 300,000 crowns, and that Brittany signed a treaty of alliance "in perpetuity" with England; this alliance was to be sealed by the marriage of the Montfortist claimant John of Montfort (son of the earlier John of Montfort) with Edward's daughter Mary.

The situation remained in stalemate for some time, with Charles of Blois as de facto Duke, but with significant territory still controlled by the Montfortists.

In 1362, when the younger John de Montfort reached 22 years of age, King Edward permitted him to return to Brittany.

Charles of Blois and Bertrand du Guesclin came to the rescue of the besieged city, but they were decisively defeated at the Battle of Auray on 29 September 1364.

This battle marked the end of this long conflict: Charles of Blois was killed and Joan of Penthièvre, finding herself a widow, saw her cause collapse.

He did not reject completely the claims of the Penthièvre family, and established the following law of succession in Brittany: King Charles V of France did not oppose the elevation of John, fearing that he might declare homage to Edward of England, his protector and former father-in-law (Mary having died in 1361).