Don Sancho, knowing of the warlike preparations of his brother, demanded explanations from him regarding their object. The answer of Don Alfonso by no means satisfied him; negotiations succeeded, becoming gradually more embittered, and in the end there was a complete rupture between Castile and Leon; the efforts of the Cid and some other honoured noblemen to prevent it having had no result.
Don Alfonso asked for aid from the kings of Navarre and of Aragon; but before they were able to afford it, Don Sancho had collected together a good army and hastened to invade the territory of his brother. The two contending parties came to blows near a village named Plantaca; they fought with great valour, and victory declared for the Castilians. The king, Don Alfonso, being conquered, and his army destroyed, was forced to retire to the city of Leon, where he intended to reinforce himself, with the object of again attacking his victorious enemies.
He encountered them again near Golpelara, on the banks of the river Carrion; another battle was fought, and, fortune changing, the Castilians were beaten, before the Cid was able to take part in the combat.
Rodrigo Diaz was very unwilling to fight against any of the children of Don Fernando, and he only decided to do so when he saw Don Sancho, whom he had accompanied in this war, quite powerless. On his arrival at the field of battle, he found the Castilian army cut up and in flight, and Don Sancho in despair. He cheered him up, assuring him that he would regain all he had lost, got together again the flying soldiers and before daybreak attacked the Leonese, who, heavy with sleep and wine, as Mariana writes, were far from thinking of such a thing. The most terrible disorder arose in the army of Don Alfonso. Some fled, others took up their arms in a careless way, all were commanding, no one obeying; they were vanquished, therefore, in a very short time. Don Alfonso, fearing that he would soon fall into the hands of his enemies, fled from the field of battle and shut himself up, with some of his followers, in the church of Carrion; the Castilians, however, surrounded it, and compelled him to surrender.
Don Sancho sent him at once to Burgos, and followed up the conquest of the kingdom of Leon. The city of that name and other towns resisted; in the end, however, they yielded, and in a few days the entire kingdom of Don Alfonso was in the hands of Don Sancho.
Many noble Castilians and Leonese, amongst whom were Doña Urraca, Peranzures, and the Cid, interceded with Don Sancho, praying him to make the condition of the prisoner as favourable as possible. The King of Castile consented to his brother going to the monastery of Sahagun, taking the habit of a monk, and renouncing the secular state.
Don Alfonso did not remain long in that monastery. Whether it was that the monastic life disgusted him, that he suspected the intentions of his brother, or that he desired to put himself in a position to recover the kingdom he had lost, whenever a favourable opportunity might present itself,—whatever was the true reason, he fled to Toledo, where he was kindly received by Almenon, who was glad to find an opportunity for fulfilling the promise which he had made to the dead king, Don Fernando, of affording the same protection to his children which he had afforded to his daughter Casilda. He told him that he might remain in his states as long as he desired; that he would provide for all his wants in such a manner that he would scarcely regret the throne which he had lost; and that he would treat him as a son. Don Alfonso entered into a covenant with Almenon to serve him in the wars in which he was engaged with other neighbouring Moors. He was accompanied by Peranzures and other cavaliers, to whom the King of Toledo made allowances, by means of which they could maintain themselves, and his ordinary occupation was the chase. For greater convenience in the pursuit of this, he built a country-house, which was the origin of the town of Brihuega.
There now only remained to Don Sancho to take possession of Zamora, in order to possess all the states which had belonged to his father. The city of Zamora was well supplied with fortifications, munitions, provisions, and soldiers, which were there in order that all emergencies might be provided for. The inhabitants were very brave and loyal, and were always ready to expose themselves to any dangers by which they might be threatened. They were under the command of Arias Gonzalo, a cavalier advanced in years, of great valour and prudence, and whose counsels, in matters of government and war, were much esteemed by Doña Urraca.
Don Sancho desired to possess that city, especially as he now held Toro, which he had taken from Doña Elvira, and, as the two were near each other, he feared that the people of Zamora, who were strong and daring, might fall upon the latter; he, however, desired to live in peace with Doña Urraca, for whom he had always felt a greater affection than for his other brothers and sisters. Hoping that he might be able to obtain Zamora in exchange for some other place, and not by force of arms, he resolved to send the Cid in order to negotiate such an exchange with the Infanta.