In 1305 Diego López V de Haro was called to appear in the Cortes of Medina del Campo, although he didn't come but after being summoned several times, to respond to the demands of his niece María I Díaz de Haro, who claimed, using the influence of her husband John of Castile, the possession of the Lordship of Biscay.

In the absence of the Lord of Biscay, John of Castile filed a lawsuit against him before Ferdinand IV, and alleged that he can prove that the Lordship of Biscay was illegally occupied by Sancho IV of Castile, which is why it was now in the hands of Diego López V de Haro, uncle of his wife.
However, while John of Castile presented the evidence to the King's representatives, Diego López V de Haro arrived, accompanied by 300 knights.
The Lord of Biscay refused to renounce to his domains, arguing that John of Castile and his wife had renounced to their rights in a solemn oath in 1300.

Failing to reach an agreement, due to the arguments presented by both parties, Diego López V de Haro returned to Biscay, although the Cortes de Medina del Campo had not yet finished until mid-June 1305.
In mid-1305, the court was in the city of Burgos, and while Diego López V de Haro proposed to appeal to the Pope, due to the solemn oath of resignation to the Lordship of Biscay made John of Castile and his wife in 1300, Ferdinand IV offered to María I Díaz de Haro the possession of several cities of the Lordship of Biscay, among them San Sebastián, Salvatierra, Fuenterrabia and Guipúzcoa, but she refused, advised by Juan Núñez II de Lara (who was distanced from with her husband) and despite the pressures of John of Castile.
Shortly thereafter, John of Castile and Diego López V de Haro signed a truce, valid for two years, during which the King was confident that the Lord of Biscay would break his alliance with Juan Núñez II de Lara.
Later, during Christmas of 1305, Ferdinand IV met with Diego López V de Haro in Valladolid, but the Lord of Biscay was accompanied by Juan Núñez II de Lara, who was forced by the King (because he was estranged from him) to leave the city, because he wished to break the alliance between him and Diego López V de Haro, although the monarch did not succeed, since the Lord of Biscay was convinced that John of Castile would not cease in his claims.

At the beginning of 1306, Lope Díaz de Haro, son and heir of Diego López V de Haro, was also estranged from Juan Núñez II de Lara and tried to persuade his father to accept the solution proposed by Ferdinand IV.
That same year, the King gave the position of First Majordomo to Lope Díaz de Haro; shortly after, the Lord of Biscay had a meeting with the King, but again caused his anger because arrived accompanied by Juan Núñez II de Lara.
During the meeting, Diego López V de Haro tried to reconcile Juan Núñez II de Lara with the sovereign, while the latter tried that the Lord of Biscay broke his relations with his ally.
Persuaded by Juan Núñez II de Lara, Diego López V de Haro left without the King's consent, while ambassadors arrived from the Kingdom of France, requesting an alliance between both countries, and also asking the hand of Infanta Isabella of Castile, sister of Fernando IV.

In April 1306, John of Castile, despite the opposition of the Dowager Queen, induced Ferdinand IV to declare war to Juan Núñez II de Lara, knowing that Diego López V de Haro would defend him, and advised the monarch besiege Aranda de Duero, where Juan Núñez II de Lara was, who, in view of the situation, broke his oath of vassalage to the King.
After a camp battle, Juan Núñez II de Lara managed to escape from the siege to which Aranda de Duero was intended to be subjected, and met Diego López V de Haro and his son, and agreed to declare the war on Ferdinand IV separately in their respective domains.
The royal troops demanded concessions from Ferdinand IV, who had to grant them even though they were not diligent in making war, and so the King ordered John of Castile to enter into negotiations with Diego López V de Haro and his supporters, to which he agreed, because his vassals didn't supp
In 1305 Diego López V de Haro was called to appear in the Cortes of Medina del Campo, although he didn't come but after being summoned several times, to respond to the demands of his niece María I Díaz de Haro, who claimed, using the influence of her husband John of Castile, the possession of the Lordship of Biscay.
In the absence of the Lord of Biscay, John of Castile filed a lawsuit against him before Ferdinand IV, and alleged that he can prove that the Lordship of Biscay was illegally occupied by Sancho IV of Castile, which is why it was now in the hands of Diego López V de Haro, uncle of his wife. However, while John of Castile presented the evidence to the King's representatives, Diego López V de Haro arrived, accompanied by 300 knights. The Lord of Biscay refused to renounce to his domains, arguing that John of Castile and his wife had renounced to their rights in a solemn oath in 1300.
Failing to reach an agreement, due to the arguments presented by both parties, Diego López V de Haro returned to Biscay, although the Cortes de Medina del Campo had not yet finished until mid-June 1305. In mid-1305, the court was in the city of Burgos, and while Diego López V de Haro proposed to appeal to the Pope, due to the solemn oath of resignation to the Lordship of Biscay made John of Castile and his wife in 1300, Ferdinand IV offered to María I Díaz de Haro the possession of several cities of the Lordship of Biscay, among them San Sebastián, Salvatierra, Fuenterrabia and Guipúzcoa, but she refused, advised by Juan Núñez II de Lara (who was distanced from with her husband) and despite the pressures of John of Castile. Shortly thereafter, John of Castile and Diego López V de Haro signed a truce, valid for two years, during which the King was confident that the Lord of Biscay would break his alliance with Juan Núñez II de Lara. Later, during Christmas of 1305, Ferdinand IV met with Diego López V de Haro in Valladolid, but the Lord of Biscay was accompanied by Juan Núñez II de Lara, who was forced by the King (because he was estranged from him) to leave the city, because he wished to break the alliance between him and Diego López V de Haro, although the monarch did not succeed, since the Lord of Biscay was convinced that John of Castile would not cease in his claims.
At the beginning of 1306, Lope Díaz de Haro, son and heir of Diego López V de Haro, was also estranged from Juan Núñez II de Lara and tried to persuade his father to accept the solution proposed by Ferdinand IV. That same year, the King gave the position of First Majordomo to Lope Díaz de Haro; shortly after, the Lord of Biscay had a meeting with the King, but again caused his anger because arrived accompanied by Juan Núñez II de Lara. During the meeting, Diego López V de Haro tried to reconcile Juan Núñez II de Lara with the sovereign, while the latter tried that the Lord of Biscay broke his relations with his ally. Persuaded by Juan Núñez II de Lara, Diego López V de Haro left without the King's consent, while ambassadors arrived from the Kingdom of France, requesting an alliance between both countries, and also asking the hand of Infanta Isabella of Castile, sister of Fernando IV.
In April 1306, John of Castile, despite the opposition of the Dowager Queen, induced Ferdinand IV to declare war to Juan Núñez II de Lara, knowing that Diego López V de Haro would defend him, and advised the monarch besiege Aranda de Duero, where Juan Núñez II de Lara was, who, in view of the situation, broke his oath of vassalage to the King. After a camp battle, Juan Núñez II de Lara managed to escape from the siege to which Aranda de Duero was intended to be subjected, and met Diego López V de Haro and his son, and agreed to declare the war on Ferdinand IV separately in their respective domains. The royal troops demanded concessions from Ferdinand IV, who had to grant them even though they were not diligent in making war, and so the King ordered John of Castile to enter into negotiations with Diego López V de Haro and his supporters, to which he agreed, because his vassals didn't supp
Ferdinand IV of Castile