In the decades following the Breton War of Succession (1341–64), the defeated faction led by Olivier de Blois, Count of Penthièvre, continued to plot against the Dukes of the House of Montfort. The Blois faction, which refused to relinquish its claim to rule over the Duchy of Brittany, had taken Duke John VI prisoner in violation of the Treaty of Guérande (1365). The 16-year-old Gilles took the side of the House of Montfort. He was able to secure the Duke's release and was rewarded with generous land grants that were converted to monetary gifts.