Infante Ferdinand de la Cerda was eldest son of King Alfonso X and heir of Castilian throne. He died before his father, and Alfonso X always stated his successor was his grandson Alfonso de la Cerda, instead of his second son, Infante Sancho. This motivated Sancho to revolt against his father. After a long civil war Sancho was finally crowned. Following an unsuccessful invasion by Aragon and Portugal, a peace settlement removed the Infante de la Cerdas from contention for the Castilian throne.
Point of Divergence
In 1258 Violante of Aragon, wife of King Alfonso X, gave birth to a son named Sancho. Deliveries were heavy, and the queen lost her ability to have children. Sancho died soon. As result, after death of Alfonso in 1284 Alfonso de la Cerda remained his only descendant in the male line and ascended to the throne as Alfonso XI