War of the Breton Succession
In the Treaty of Guérande, which ended the War of the Breton Succession, the Duchy of Brittany was made hereditary in the males of the Montfort line, at the termination of which it was to pass to the males of Blois-Penthièvre. In 1420 the Penthièvre family imprisoned the duke in a failed attempt to regain Brittany. When the last male Montfort duke, Francis II of Brittany, died, the duchy passed to his daughter, Anne of Brittany.