The House of Montfort-Britanny was a Breton-French noble family, which reigned in the Duchy of Brittany from 1365 to 1514. It was a cadet branch of the House of Dreux; it was thus ultimately part of the Capetian dynasty. It should not be confused with the older House of Montfort which ruled as Counts of Montfort-l'Amaury.
It succeeded the Brittany branch of the House of Dreux, invoking already in 1341 a right to succeed John III, Duke of Brittany. A war ensued, ultimately won by Montforts in 1364.
The dynasty was succeeded by Valois family, first Claude, the daughter of Montfortine duchess Anne, and then Claude's sons. Already from the time of Duchess Anne's marriage, however, the duchy was gradually subsumed to the French state, in practice, so it can be said that French central government succeeded the Montforts.
Rise to power
Count John of Montfort (1365 - 1421) was the sole surviving son of Yolande of Dreux, Countess of Montfort suo jure (and dowager queen of Scotland) from her second marriage to Arthur II, Duke of Brittany. John inherited in 1322 Montfort-l'Amaury from his mother. However, he was only a younger son of the Duke, who had several older sons from his first marriage. John only received some appanage in Brittany, and his maternal inheritance, a countship, was a much more important possession.
However, his eldest half-brother, John III, Duke of Brittany, was childless in spite of his three marriages. Duke John III died in 1341, and his duchy's nobles proclaimed Countess Joan of Dreux reigning duchess. She was the daughter of the late Guy, Count of Penthièvre, Duke John III's younger full brother, and thus John III's full niece.
John of Montfort however invoked both the principle of Salic law (allowing only males to succeed) and the principle of proximity of blood (as used earlier, e.g. by John of England against Arthur I of Brittany), having himself proclaimed Duke. This led to the Breton War of Succession, a part of the Hundred Years' War. His patron in this quest was king Edward III of England. The rivals, Duchess Joanna and her husband Charles of Blois were supported by the Valois kings of France. John of Montfort died without accomplishing his objective of becoming sole ruler of Brittany, but his wife Joanna of Flanders continued the fight in the name of their son John IV, Duke of Brittany (1339/40–99) who ultimately got the upper hand in the strife.
In the midst of the conflict, in 1352, the Estates of Brittany (États de Bretagne) were established. They would develop into the Duchy's parlement.
Ruling as dukes
When the peace was sealed in 1365, it was stipulated that the Montfort branch would succeed in Brittany subject to the restrictions of Salic law and in the case of their male line going extinct, the heirs of Joanna of Penthièvre will succeed the last male Montfortist duke. The Breton ducal house and many Breton noble families had followed a semi-Salic tradition which permitted a daughter to inherit from her father. The Blois-Penthièvre family received more estates in Brittany as partial compensation.
Brittany retained its autonomy, or rather independence, although continuously giving lip service to French sovereignty. After the Breton War of Succession, Brittany still had links with the English through the Earldom of Richmond, until the Wars of the Roses.
John IV, Duke of Brittany was deserted by his nobles in 1373 and left for exile in England. Louis I, Duke of Anjou, brother of Charles V of France, and a son-in-law of the deposed Penthièvre Duchess Joanna, was appointed lieutenant-general of Brittany by the king, who in 1378 sought to annex Brittany to France, which provoked the Bretons to recall John IV from exile.
The second Treaty of Guérande (1381) established Brittany's neutrality in the Anglo-French conflict, although John continued to make homage to King of France.
In 1420, John V, Duke of Brittany was kidnapped by Olivier de Blois, count of Penthièvre, son of Joanna of Penthièvre. John's wife, Joan of France besieged the rebels and set free her husband, who confiscated the Penthièvre's goods.
According to the succession order enacted, in 1457 Duke Peter II was succeeded by his elderly uncle Arthur de Richemont instead of his sister Isabelle de Bretagne-Montfort (who married into the Laval family and from whom the future Chabot branch of the Rohan family descends).
In 1465, Francis II took the county of Penthièvre from its heiress, Nicole de Bretagne-Blois, thus again undermining the rival family's position in Brittany.