As such, he embodied the claim of his paternal grandfather, Infante Ferdinand de la Cerda, eldest son of King Alfonso X the Wise, while being heir to both the Lara holdings of his namesake maternal grandfather, Juan Núñez de Lara "the Fat", and to the lordship of Viscaya coming from his maternal grandmother Teresa Díaz of Haro.

Therefore, Don Juan Manuel, suggested that the Lord of Lara be married to María Díaz II de Haro.

With this, Don Juan Manuel got the support of the Portuguese ruler who had been allied with Alfonso XI, soon to be joined by the Sultan of Granada.

The agreement with the ruler of Granada was made with the intention of pressuring Alfonso XI to return María Díaz II de Haro's possessions that had been confiscated by him.

Alfonso XI delayed the granting of a response to such demands, and shortly thereafter went to meet with Don Juan Manuel in Peñafiel.

Meanwhile, the city of Gibraltar capitulated to the Muslims and Alfonso XI began to besiege it, but had to raise the siege before the arrival of troops from Granada and Algeciras.

During Lent of 1334, a squire of Juan Núñez de Lara gave Alfonso XI a letter, in which he informed the king that he was ending his vassal relationship with him.

Alfonso XI, after leaving some of his troops besieging the Peña de San Juan, he returned to Castile, where in the town of Agoncillo, ordered the execution of Juan Alfonso de Haro, for abuses committed by him in the kingdom, for his support of rebel barons, and for having appropriated funds belonging to the Crown.

In order to terminate disputes, Alfonxo XI confirmed the lordship of Biscay on Juan Núñez de Lara.

Juan Núñez de Lara agreed, in the future, to recognize Alfonso XI as his king and give him all that he required.

The village of Torrelobatón soon capitulated to the king's troops, with Alfonso XI imposing the condition that they never return to power of Juan Núñez de Lara.

Juan Núñez de Lara, finding himself surrounded and without possibility of receiving relief, agreed to negotiate peace with Alfonso XI.

In 1339, the ambassadors of King Peter IV of Aragon reported to Alfonso XI that they were willing to help in the fight against Muslims in the south of the Iberian Peninsula.

In 1341 Alfonso XI conquered the town of Alcalá la Real, Juan Núñez de Lara was present lieutenant of the king.

In 1349, after spending several years in retirement, Juan Núñez de Lara was summoned by Alfonso XI, together with the other nobles, so they could assist in the siege of Gibraltar.

The Castilian nobles, including Juan Nunez de Lara, petitioned Alfonso XI to lift the siege, because he risked losing his life if he persisted in the company.

Juan Núñez de Lara, Ferdinand, Prince of Asturias and the other nobles lifted the siege of Gibraltar and led the body of Alfonso XI to the town of Seville, where he was buried in the Chapel Royal.