His wife's family, the House of Penthièvre, were descendants of the House of Dreux as Dukes of Brittany. The House of Penthièvre had lost the dukedom of Brittany to the House of Montfort in the Breton War of Succession in the 14th century. They had subsequently attempted to overthrow the Montfortist Dukes, with no success. Invoking the alleged hereditary rights of his wife, Mercœur endeavoured to make himself independent in that province, and organized a government at Nantes, calling his son "prince and duke of Brittany".