War of the Breton Succession
Joan was one of the protagonists of the War of the Breton Succession. The issue of succession to the ducal crown would involve the issue of whether a child could, regardless of gender, claim the right of "representation" of a deceased parent — in which case Joan would inherit her father's rights as the second brother of the late duke of Brittany — or whether the next eldest male heir in a partially parallel lineage outranked all others. In the Breton succession, the collateral claimant was Joan's half-uncle John, Count of Montfort, born from the second marriage of Joan's grandfather Arthur II, Duke of Brittany to Yolande of Dreux, Queen of Scots. Joan's uncle John III had been alienated from Yolande, his stepmother, and sought to prevent his half-brother John from succeeding him, which included an abortive attempt to annul his father's second marriage and so render his half-siblings illegitimate.