[1] Alfonso took part in the Reconquista, conquering several territories within Extremadura, capturing Mérida and Badajoz in 1230, which allowed for the eventual conquest of Seville during the reign of his son Ferdinand III.

[4] The convening of the Cortes de León in the cloisters of the Basilica of San Isidoro would be one of the most important events of Alfonso's reign.

The difficult economic situation at the beginning of his reign compelled Alfonso to raise taxes on the underprivileged classes, leading to protests and a few town revolts.

In response, the king summoned the Cortes, an assembly of nobles, clergy and representatives of cities, and subsequently faced demands for compensatory spending and greater external control and oversight of royal expenditures.

Alfonso's convening of the Cortes is considered by many historians, including John Keane,[5] to be instrumental to the formation of democratic parliaments across Europe.

In spite of the democratic precedent represented by the Cortes and the founding of the University of Salamanca, Alfonso is often chiefly remembered for the difficulties his successive marriages caused between him with Pope Celestine III.

[10] The annulment of this marriage by the pope drove the younger Alfonso to again attack his cousin in 1204, but treaties made in 1205, 1207, and 1209 each forced him to concede further territories and rights.

[13] The Pope was, however, compelled to modify his measures by the threat that, if the people could not obtain the services of religion, they would not support the clergy, and that heresy would spread.

[18] The marriage was annulled five years later, on grounds of consanguinity but three children had been born by then, being: On 17 November 1197, Alfonso IX married his first cousin once removed, the infanta Berengaria of Castile.

[29] Alfonso IX and Estefanía were the parents of possibly two sons, but definitely of one, being:[b] According to Spanish historian Julio González, after his relationship with Estefanía, the king had a lover from Salamanca, of unknown origin, whose name was Maura and with whom he had issue:[31] Alfonso also had a relationship with a noblewoman from Portugal, Aldonza Martínez de Silva, daughter of Martim Gomes da Silva and his wife Urraca Rodrigues,[32] which lasted from 1214 to 1218.