According to the Muslim historian Ibn al-Athir, the Diet of Paderborn had received the representatives of the Muslim rulers of Zaragoza, Girona, Barcelona and Huesca. Their masters had been cornered in the Iberian peninsula by Abd ar-Rahman I, the Umayyad emir of Cordova. These "Saracen" (Moorish and Muwallad) rulers offered their homage to the king of the Franks in return for military support. Seeing an opportunity to extend Christendom and his own power, and believing the Saxons to be a fully conquered nation, Charlemagne agreed to go to Spain.

In 778, Charlemage led the Neustrian army across the Western Pyrenees, while the Austrasians, Lombards, and Burgundians passed over the Eastern Pyrenees. The armies met at Saragossa and Charlemagne received the homage of the Muslim rulers, but the city did not fall for him. Indeed, Charlemagne faced the toughest battle of his career. The Muslims forced him to retreat, so he decided to go home, as he could not trust the Basques, whom he had subdued by conquering Pamplona. He turned to leave Iberia, but as his army was crossing back through the Pass of Roncesvalles, one of the most famous events of his reign occurred: the Basques attacked and destroyed his rearguard and baggage train. The Battle of Roncevaux Pass, though less a battle than a skirmish, left many famous dead, including Roland.