His elder brother, Ferdinand de la Cerda, died in November 1275, and in 1282 Sancho assembled a coalition of nobles to declare for him against Ferdinand's son Alfonso, then took control of the kingdom when Alfonso X died in 1284. This was all against the wishes of their father, but Sancho was crowned in Toledo nevertheless. Sancho's ascension was in part due to his rejection of his father's elitist politics. Sancho was recognised and supported by the majority of the nobility and the cities, but a sizable minority opposed him throughout his reign and worked for the heirs of Ferdinand de la Cerda. One of the leaders of the opposition was his brother John, who united to his cause the lord of Biscay, Lope Díaz III de Haro. Sancho responded by executing the Lord of Biscay and incarcerating his brother. According to the chroniclers, he cemented his hold on power by executing 4,000 other followers of Infante Alfonso, son of Ferdinand de la Cerda, in Badajoz. He executed 400 more in Talavera and more in Ávila and Toledo. Upon dispensing with this opposition, Sancho pardoned his brother, who was released. John bided his time before fomenting revolt again: the conflict over Tarifa. He called in the aid of the Marinids in Morocco and besieged Guzmán the Good in his castle. At this siege occurred that famous act of heroism, the innocent death of the son of Guzmán. Tarifa was faithfully defended until Sancho could rescue it and the Marinids retreated to the Maghreb. The intent of both John and the Sultan of Marinids was foiled. When James II succeeded to the Crown of Aragon, he endeavoured to bind the two crowns more closely and to unite in the Reconquista. Indeed, both of James' predecessors had tried to do likewise. Sancho was also the friend and tutor of Juan Manuel of Castile. Just before succumbing to a fatal illness he appointed his wife, María de Molina, to act as regent for his nine-year-old son, Ferdinand IV. He died in 1295 in Toledo.
Family
Sancho married Maria de Molina in 1282, but at first their marriage did not have the necessary papal dispensation for two reasons: First, they had a distant blood relation, and second, Sancho had been betrothed as an infant to a rich Catalan heiress named Guillerma Moncada. They had the following children:
