King of Castile (1217-1252) and Leôn (1230-1252). Ferdinand III inherited the kingdom of Castile on the abdication of his mother, Queen Berenguela, daughter of King Alfonso VIII of Castile. In 1230 he succeeded his father, Alfonso IX, in the kingdom of Leôn. From this point on both kingdoms remained united.
King Ferdinand is associated with the culmination of the offensive of Christian Spain against Islam. His plan for the reconquest of the country adopted the form of a crusade, thanks to the support of the papacy. However, from this time the crusade in the Iberian Peninsula developed specific characteristics that transformed it into a further expression of the strength of the royal power. The Christian offensive was aided by the increasing disintegration of the Almohad Empire, which gave rise to a fragmentation and weakening of political power among the Muslims of the peninsula. Its principal successes were the conquest of Cordoba (1236), Murcia (1243), Jaén (1246), and, finally, Seville (1248). As a consequence, al-Andalus was reduced to one independent kingdom, Granada, and two small protectorates, Niebla and Murcia. These three states were made subject to Castile, obliged to make tribute payments (Sp. parias). Military successes brought considerable territorial gains: the ancient kingdoms of Castile and Leôn, amounting to some 250,000 square kilometers (96,500 sq. mi.), increased by a third with the incorporation of Andalusia and Murcia, some 100,000 square kilometers (38,600 sq. mi.).
This new situation permitted the kingdom of Castile and Leôn to consolidate its power throughout the entire peninsula, and allowed Ferdinand III to establish the basis for a solid monarchy. The foundations on which this monarchy rested were of an extremely diverse nature, but four main elements can be distinguished: the unification of laws throughout its territory, submission to the church, an alliance with the nobility, and unequivocal support for the military orders. Ferdinand III extended the frontier law peculiar to the fuero (foundation privilege) of Cuenca to numerous towns and cities that had been recently incorporated; he revived the Visigothic law, or Fuero Juzgo, which held the authority of the king in the highest esteem; he was also responsible for the creation of a common judicial corpus for the whole kingdom, which was implemented by his son Alfonso X. The king also contributed decisively to increasing the power of the church: the offensives of the reconquest were accompanied by numerous restorations of old diocesan churches, which received generous grants; in return, the monarch demanded loyalty and submission from his bishops. The king was also generous in his treatment of the noble lineages; thanks to the victorious war against Islam, they experienced an increase in their patrimonial fortunes, and consequently few conflicts between Crown and nobility occurred during his reign. Finally, Ferdinand III gave strong support to the military orders, granting them extensive estates in the region between the River Tagus and the Sierra Morena; in return the orders provided efficient military collaboration, and they colonized the territory by means of a profitable livestock economy. Ferdinand III was canonized by the church during the Counter-Reformation of the seventeenth century. He was succeeded by his son Alfonso X.