When King Henry IV died in 1474 Isabella proclaimed herself successor queen, disputing the right of Juana la Beltraneja (the king's 13-year-old daughter and her niece) to ascend the throne. During the ensuing civil war between the followers of Isabella and Juana, there was also conflict with Portugal since King Alfonso V of Portugal sided with his niece Juana. Gonzalo fought for Isabella under Alonso de Cárdenas, grand master of the Order of Santiago. In 1479 he fought in the final battle against the Portuguese leading 120 lancers. Cárdenas praised him for his service. When the war ended Isabella and her husband Ferdinand were the rulers of Castile and Aragon.