He was one of the most powerful men in Castile, as evidenced by his numerous titles and the offices he held, and was involved in much of the kingdom's most important political and military events, notably in the various conflicts between the nobility and the candidates for succession to the throne that would culminate in the War of the Castilian Succession and that would only calm down with the final recognition of the Catholic Monarchs, whom he initially opposed but eventually supported.

Álvaro de Zuñiga is one of the main characters in the so-called Farce of Ávila, a ceremony that took place on 5 June 1465, during which the deposition of Henry IV was staged and D. Alfonso was acclaimed king.

[20] The relations of Álvaro de Zúñiga, Count of Plasencia, with King Henry IV and with the Favourite Juan Pacheco, Marquis of Vilhena, were very cordial and firm between 1458 and 1464.

[6] The members of the league, meeting in Yépez in February 1461, drafted a political program with four points, among which were respect for the privileges of the nobles and the oath of Alfonso as Prince of Asturias and heir to the throne of Castile.

On 26 August 1461 Henry IV signed an agreement with the league that satisfied the nobility, as it met their wishes for the maintenance of the oligarchy and the institution of a government run by a team of nobles.

The Marquis of Vilhena, Juan Pacheco, sends royal troops to arrest the Archbishop of Seville, Alonso de Fonseca, who seeks refuge in the castle of Béjar, owned by the count of Plasencia.

Álvaro de Zúñiga breaks the agreed truce and mobilizes his troops to help the archbishop, which forces the lifting of the siege ordered by Henry IV on the Fonseca estates of Coca and Alaejos.

The manifesto of the assembly of Burgos called for the removal of Beltrán de la Cueva, stated that Joanna was not the daughter of Henry, and demanded that Prince Alfonso be recognized as the successor, that he be given the title of master of the Order of Santiago, and that he be raised in the house of the counts of Plasencia.

The nobles gather in the city outside the walls, and place on a wooden platform a dummy dressed in mourning, with a crown, cloak, sword, and scepter, representing Henry IV.

At the end of November, the Zúñiga manage to get the count of Arcos, the duke of Medina Sidonia, and the Sevillian municipality to solemnly swear allegiance to Alfonso XII.

Diego López de Zúñiga is appointed Corregedor of Seville, but the city managed to impose its conditions and eventually prevented him from taking office.

In August 1466, the moderate rebel nobles decided to reconstitute the old league and bring together the count of Plasencia and the duke of Medina Sidonia, who had recently become estranged.

[45] Alvaro de Zúñiga decided to accept the peace plan of the archbishop of Seville at the end of 1467 and offers his palace as a safe asylum for the king.

Henry IV arrives in Plasencia on 28 December 1467, being welcomed by Álvaro de Zúñiga, and enjoys his support and hospitality for four months, promising to offer the village of Trujillo to the count.

It was determined that Isabella's marriage needed prior authorization from the king and the advice of the marquis of Vilhena, the archbishop of Seville, and Álvaro de Zúñiga.

[49] In a letter of 2 May 1469 the Portuguese king offered the expansion of the houses of Zúñiga, Fonseca, Pacheco, Velasco, and Mendoza in return for fidelity, political and military support.

[50] On 19 October 1469 Isabella marries in Valladolid, without royal consent, Prince Ferdinand, King of Sicily and successor to the throne of Aragon, who undertakes to assist his wife in her succession rights.

[54] In the summer of 1470, Henry IV confirmed the title of duke of Arévalo and granted an annual income to Álvaro de Zúñiga in return for Trujillo not being incorporated.

[56] Álvaro de Zúñiga took part in the meeting between the king of Castile and León, Henrique, and Afonso V of Portugal, which took place in May 1472 on the Caia river, between Badajoz and Elvas.

[59] The House of Zúñiga had heavily armed possessions in Béjar, Plasencia, Peñaranda de Duero, Arévalo, and Burgos, making them omnipotent in this area.

On 20 February 1472 Pope Sixtus IV granted the degree of master of Alcántara to Juan de Zúñiga y Pimentel, then 13 years old.

The duke of Arévalo and the marquis of Vilhena took Princess Joanna from Madrid to Trujillo, where she was proclaimed as the legitimate successor of Henry IV in March 1475.

The Catholic Monarchs were celebrating a great tournament in Valladolid with the presence of the high nobility when, on 3 April 1475, they received a message from the king of Portugal, that was in reality a declaration of war.

The House of Zúñiga, unhappy with the little support provided by the king of Portugal, who let the castle of Burgos fall without risking a short-distance march, decided to suspend the fight and remained in strict neutrality from January 1476.

[72] The main points of this agreement were: The Duke of Arévalo castellanizes his family name to Zúñiga, replacing the previous forms Stunica, Stúñiga, and Estúñiga.

On 1 January 1480 the Catholic Monarchs granted Álvaro de Zúñiga y Guzmán the titles of duke of Plasencia and count of Bañares, which would be ratified on 16 February 1506.

The construction of the church and convent began in 1477, and on 13 April 1487 the complex was blessed by Friar Pedro de Villalobos, the apostolic visitor of the diocese of Plasencia.

The dukes, cultured figures of the Renaissance, also founded the first college in Extremadura to remedy the reputation of ignorance of the men of the region, more skilled in arms than in letters.

[86] The Catholic Monarchs confirmed by royal decree of 10 June 1488, authorizing the duke of Plasencia and Béjar to declare his grandson Álvaro as successor to his house, titles, and states.

[89] He was declared the first fnight of the Realm by Henry IV on 3 May 1464,[90] and alcalde of the fortress of Burgos,[91] administrator of the grand master of the Order of Alcántara,[92] 2nd count of Plasencia[93] and 1st duke of Arévalo in 1469.