Since the second half of the eleventh century, the kings of Aragon and the counts of Barcelona and Urgell had pressed with stubborn persistence against the Muslim towns and fortresses of the Marca Superior, the Upper March of al-Andalus. Their eyes were fixed on the lowlands along the Segre and Cinca rivers, a fertile, prosperous corridor that ran all the way to the mouth of the Ebro and, beyond it, the Mediterranean. Four towns anchored the region: Lleida, Mequinenza, Fraga, and Tortosa. Take them, and Aragon would reach the sea. Fraga was one link in that chain, and Alfonso meant to have it. He had made a career of exactly this kind of frontier warfare, wearing down Muslim strongholds one by one, and few opponents had ever made him pay for his boldness. He brought his army down from Aragon and closed his grip around the town, confident that hunger and pressure would do the rest.

The Almoravid answer was swift and coordinated. From Cordoba, the emir, a son of the caliph, fielded two thousand horsemen. The emir of Murcia and Valencia, Yahya ibn Ghaniya, added five hundred more, and the governor of Lleida contributed another two hundred. Combined under the command of Ibn Iyad, the governor of Lerida, this relief force rode hard for the besieged town. When it appeared on the morning of 17 July, the defenders inside Fraga saw their chance and sallied out from the gates. Alfonso, still trusting in what he judged to be his numerical and tactical advantage, did not flinch. He rallied his men to meet both threats at once, the sortie in front and the relief on his flank. Between the town's fighters and the relief column he now faced thousands of fresh horsemen on open ground, yet a lifetime of narrow victories had taught him to trust his own judgment over the arithmetic.

The king's own entourage crashed into the cavalry of Yahya ibn Ghaniya, and there the day turned. Almoravid horsemen cut the Aragonese ranks apart. Sensing the collapse, the townspeople poured out of Fraga and fell upon the lightly guarded Christian camp, killing the soldiers left behind and carrying off the army's entire store of provisions back behind the walls. Then the emir of Cordoba launched his cavalry in a final charge and shattered what remained of the Christian line. Alfonso, who had won so often by holding his nerve, had nothing left to hold. Stripped of most of his soldiers, the Battler turned and rode north for the safety of Zaragoza, a beaten man on a road he had traveled many times in triumph.

The toll fell heaviest on men who had followed the king across the Pyrenees. Among the dead were French knights of real standing: Aimery II of Narbonne, Bertran de Risnel, and Centule VI, Viscount of Bearn, who had marched with Alfonso as an ally rather than a subject. Guy of Lescar was taken alive and carried into captivity at Valencia. These were not anonymous casualties but named lords, each with lands and families who would soon receive word that no one wanted to bring. The king survived the field but not the wound to his cause. Alfonso I of Aragon, the Battler who had fought a lifetime of battles, died on 7 September 1134, less than two months after the worst of them. The Mediterranean he had reached for stayed out of Christian hands for another generation, and Lleida itself would not fall until 1149.