The king departed from Monção and went to Lisbon, where he left the queen, because he intended to besiege Campo Maior, an important town in his kingdom between the Tagus and Guadiana [rivers], which supported the King of Castile. Its governor was Gil Vasques de Barbuda, a cousin of the Master of Alcántara, Dom Martim Eanes [de Barbuda]. On 1 September, the king arrived in Estremoz with his men and the constable and decided in his royal council first to lay siege to Olivença, held by Pero Rodrigues da Fonseca in support of the king's enemies. When Pero Rodrigues heard this, he made it known to the king that he wished to go over to him and pledge the place to him. To Olivença the king sent Afonso Vasques Correia, the Commander of Ortalagoa, and Gonçalo Lourenço, his private secretary, to confirm with Pero Rodrigues the message he had sent to the king. Once he made his promises, though with no wish to keep them, those sent to Olivença by the king returned to Estremoz. The latter immediately departed and went to lay siege to Campo Maior, arriving at that town on 15 September.

While he was laying siege to the town, Prince João, who was in Castile at the time, arrived in Olivença and was received in the town by Pero Rodrigues. Pero Rodrigues failed to keep the pledge that he had given to the king; indeed, he had only done that in order to prevent the king from attacking him. At that point, many troops from all parts of Andalusia arrived in Badajoz, led by the Master of Santiago and the Master of Calatrava. Martim Afonso de Melo heard of the arrival of these troops and, in order to be certain about it, he left the camp at midnight and went to set up an ambush a league from Badajoz. When dawn came, he took charge of the watch. As morning broke, he saw as many as eighty horsemen emerging from Badajoz; these men took a look at the siege camp then turned back. He advanced to confront them and they began to flee. The Portuguese knocked some of them off their horses and the others withdrew into Badajoz.