The Castilian Civil War of 1437–1445 was a civil war in which two noble factions fought for power in the Crown of Castile. On one side was Constable Álvaro de Luna, King John II of Castile, and Henry Prince of Asturias. On the other side was the noble League led by Infantes of Aragon John and Henry, sons of Ferdinand of Antequera, who was the regent of Castile during the minority of John II. Although the Infantes of Aragon faction won in 1441, imposing their conditions in the Medina del Campo ruling, the final victory went to the royalist faction and the Constable, who won the decisive battle of Olmedo. According to historian Carme Batlle, Álvaro de Luna is primarily responsible for the war. Following his victory over the Infantes of Aragon during the truces of Majano, Luna's "authoritarian excesses" escalated and led to a civil war. Although Batlle places the war's start in 1439.
Arrest of Pedro Manrique and Castronuño agreement (1437-1439)
After the defeat of the Infantes of Aragon in the Castilian-Aragonese War of 1429-1430, the position of the Constable Álvaro de Luna at the Castilian court was consolidated, but after a few years a faction of the nobility began to oppose the almost absolute power that he had achieved thanks to King John II's trust in him. One of the leaders of the group was Pedro Manrique, the Adelantado Mayor of León. On August 13, 1437, he was apprehended in Medina del Campo on the king's orders, at Álvaro's request. This event sparked a period of aristocratic discord in Castile that Jaume Vicens Vives stated escalated into a civil war. Following the arrest of Pedro Manrique, his supporters and relatives rose in rebellion alongside nobles who opposed Álvaro de Luna. These nobles were led by Fadrique Enríquez, the Admiral of Castile, as well as the Counts of Benavente, Ledesma, and Valencia de John, namely Rodrigo Alonso Pimentel, Pedro de Estúñiga, and Pedro de Acuña y Portugal, respectively.
In August of 1438, Pedro Manrique escaped from confinement in Fuentidueña de Tajo Castle. In February 1439, he and Castilian Admiral Fadrique Enriquez signed a letter to King John II demanding the banishment of Constable Alvaro de Luna's court "with all his relatives and people, so that Your Grace may remain in all of your free power." In the next month, the rebellious aristocrats surprised Valladolid, prompting King John II to seek assistance from the Infantes of Aragon, including John, the Navarrese king consort. He obtained royal permission to cross the border in accordance with the stipulations of the Concord of Toledo of 1436. On April 6, 1439, he convened with the king in Cuéllar, where he met with the court and the primary unit of the Constable's army. He was joined by his brother, Archbishop of Toledo Juan de Cerezuela, and a group of nobles who supported him, including the Master of the Order of Alcántara Gutierre de Sotomayor, the Count of Haro Pedro Fernández de Velasco, and the Bishop of Palencia, Gutierre Álvarez de Toledo. John was accompanied by his brother, Infante Henry. However, Henry soon joined the side of the revolted nobles when they promised him the return of all his possessions confiscated at the end of the Castilian-Aragonese War of 1429-1430, as well as the mastery of the Order of Santiago, which he had previously held.
In June 1439, John, King of Navarre, convened leaders from two opposing factions in Tordesillas. The purpose was to negotiate an agreement without resorting to armed conflict. On one side were King John II himself, the Constable Álvaro and the Count of Castro; on the other, the Admiral of Castile, the Adelantado Mayor of León, the Count of Benavente and the Comendador Mayor of Castile. No agreement was reached, as the nobles supporting Álvaro de Luna refused to return the goods they had confiscated from the Infantes of Aragon in 1430. A skirmish occurred a few days after the unsuccessful Tordesillas meeting in Roa, where the armies of the Count of Ribadeo, belonging to Álvaro de Luna's faction, clashed with the rebel nobility who had formed a League. However, the discussions did not come to an end, and a temporary resolution was reached in October 1439, referred to as the Castronuño agreement, that inflicted a six-month banishment from the court on Álvaro de Luna.