The loot. The army was retreating to France after causing significant damage to the Basques with the destruction of Pamplona. Irreparable damage and losses, apart from any looting of valuables and money. All of this was irreparable and irreversible given the power of the invader. They were also retreating with the loot captured Zaragoza and Pamplona. There are indications that Charlemagne had blackmailed the Muslim walis, forcing them to hand over considerable fortunes and ages: ...obsidione itaque cincta Caesaraugustana civitate, territi Sarraceni obsides dederunt, cum inmense pondere auri. (Ann. Mettens. ad ann. 778). ...with Zaragoza besieged, the terrified Saracens gave them ages and a huge amount of gold. And with reverterentur with preda magna. (Ann. Anniani). And when he was retreating with his immense booty. It is to be assumed that the peoples of the Ebro basin, Basques and Muslims, would have paid from their own money the costs of occupation and maintenance of the Frankish army and the Frankish garrison of Pamplona.