King Pedro was keen to return to war against Aragon, declaring that the peace which he had reached had been against his will, owing to his fears about the Red Emir. He therefore formed an alliance with the King of Navarre, whereby they agreed to become allies and help each other, and then commanded his forces to be in readiness. No one, however, thought that he planned to attack Aragon, owing to the peace which existed between them. Secretly, before the King of Aragon could find out, and in order to seize a number of townships in the meantime, he invaded Aragon and captured six castles and surrounded the town of Calatayud. While laying siege to it, he seized thirteen castles in that region.

The King of Aragon, who was at the other end of his kingdom, was astounded when he heard what had taken place. He appealed for help from Provence, where Count Enrique, his brothers and the other exiled Castilian noblemen were at war, undertaking to grant them great rewards when in his kingdom.

Meanwhile, the town of Calatayud endured so harsh a siege that it was easy for King Pedro to take it through negotiation. He left a garrison there and made his way back to Seville. As he feared the wrath of the King of France, owing to the death of his wife Queen Blanche, whom he had ordered to be killed, he now entered into a very firm alliance with King Edward of England and with the Prince of Wales his son, according to which they would help one another against all others.

He immediately went back into Aragon, reached the town of Calatayud, which was already on his side, and seized seven villages around it. When he forced his way into Cariñena, he ordered the killing of all the inhabitants, till there was not a single one left. It is said that he ordered them all to be killed because, after he had laid siege to it and found that he could not capture it, he had raised the siege, whereupon the inhabitants, seeing his men leaving, began, much as they pleased, to shout all manner of insults and curses from the ramparts. The king was infuriated at this and commanded his troops to make their way back to the town.