Joan married John V, Duke of Brittany, in 1396. Three years after the wedding, her spouse became duke and she duchess of Brittany.
As duchess, Joan is perhaps most known for her role during the conflict between John V and the Counts of Penthièvre. The Penthièvre branch had lost the Breton War of Succession in the 1340s. As a result, they lost the ducal title of Brittany to the Montforts. The conclusion to the conflict took many years to confirm until 1365 when the Treaty of Guérande was signed. Despite the military loss and the diplomatic treaty, the Counts of Penthièvre had not renounced their ducal claims to Brittany and continued to pursue them. In 1420, they invited John V to a festival held at Châtonceaux. He accepted the invitation, but when he arrived, he was captured and kept prisoner.
The Counts of Penthiève then spread rumours of his death, and moved him to a new prison each day. Joan of France called upon all the barons of Brittany to respond. They besieged all the castles of the Penthièvre family one by one. Joan ended the conflict by seizing the dowager countess of Penthièvre, Margaret of Clisson, and forcing her to have the duke freed.
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