Following the sealing of this alliance at Paderborn,[11] Charlemagne marched across the Pyrenees in 778 "at the head of all the forces he could muster".[12] His troops were welcomed in Barcelona and Girona by Sulayman al-Arabi.[13] As he moved towards Zaragoza, the troops of Charlemagne were joined by troops led by Sulayman.[12] Husayn of Zaragoza, however, refused to surrender the city, claiming that he had never promised Charlemagne his allegiance. Meanwhile, the force sent by the Baghdad caliphate seems to have been stopped near Barcelona.[14] After a month of siege at Zaragoza, Charlemagne decided to return to his kingdom.[14] On his retreat, Charlemagne suffered an attack from the Basques in central Navarra. As a reprisal he attacked Pamplona, destroying it.[14] However, on his retreat north his baggage train was ambushed by the Basques at the Battle of Roncevaux Pass on August 15, 778.[15]