This attack took the Franks by surprise due to its scale, significantly larger than any they had encountered on their journey from Pamplona to Errozabal. The Saxon Poet describes the scene: "baffled by the sudden tumult, panic spread in the [Frankish] army" (pavor… subitoque tumulto turbantur). Similarly, the Annales qui dicuntur Einhardi highlight that the Basques "caused great chaos throughout the entire army" (totum exercitum magno tumultu perturbant).
Contrary to some accounts, the primary sources do not indicate an attack on the rearguard per se, but rather on the rear portion of the supply caravan and the guard corps protecting it, which impacted the entire Carolingian force. That is, the attack impacted not just the rearguard but the entire army. These sources also cite "the difficult terrain" (iniquitate locorum) and "the disparity in combat styles" (genere inparis pugnae) as factors that disadvantaged the Franks in this critical and decisive encounter.
Einhard, in the Vita Karoli Magni, notes that the Basques managed to catch the Carolingian troops off guard by launching a downhill assault (desuper incursantes), plunging into the army's column and thus initiating close-quarters combat where their lighter armor and weapons provided a tactical benefit.
