War of the Breton Succession
When John III died a dispute emerged regarding the inheritance of the Duchy of Brittany. John III had tried to prevent the inheritance of the duchy by his half brother John of Montfort. Joan and Charles claimed the ducal crown and were subsequently proclaimed the Duchess and Duke of Brittany, which John of Montfort also claimed. Their dispute prompted the War of the Breton Succession. In 1352, during that war, the Estates of Brittany were established. In 1364, Charles died and John IV, Duke of Brittany, John of Montfort's son, was confirmed as duke. Joan was permitted to keep Penthièvre and use the title Duchess of Brittany until her death.