In September 1479, Portugal and the Catholic Monarchs of Aragon and Castile resolved major issues between them through the Treaty of Alcáçovas, including the issue of Isabella's rights to the crown of Castile. Through close cooperation, the royal couple were successful in securing political power in the Iberian peninsula. Ferdinand's father had advised the couple that "neither was powerful without the other". Though their marriage united the two kingdoms, leading to the beginnings of modern Spain, they ruled independently and their kingdoms retained part of their own regional laws and governments for the next centuries.