supporters of the Umayyad Caliphate were established firmly in southern Spain under Abd ar-Rahman I, and constituted a strategic threat both to the Carolingians on their southern border, and to the Abbasids at the western end of their dominion. Embassies were exchanged both ways, with the apparent objective of cooperating against the Umayyads of Cordova (Al-Andalus): a Frankish embassy went to Baghdad in 765, which returned to Europe after three years with numerous presents, and an Abbasid embassy from al-Mansur visited France in 768. Commercial exchanges Commercial exchanges occurred between the Carolingian and Abbasid realms, and Arabic coins are known to have spread in Carolingian Europe in that period. Arab gold is reported to have circulated in Europe during the 9th century, apparently in payment of the export of slaves, timber, iron and weapons from Europe to Eastern lands. It is noted that Charlemagne made attempts to establish an open market between the Carolingians and Abbasids, possibly as a means for the Carolingians to economically benefit from trade with the Abbasids. As a famous example, the 8th century English king Offa of Mercia is known to have minted copies of Abbasid dinars struck in 774 by Caliph Al-Mansur with "Offa Rex" centered on the reverse amid inscriptions in Pseudo-Kufic script. Charlemagne and Harun al-Rashid Strategic interest in Spain (777–778) In 777, pro-Abbasid rulers of northern Spain contacted the Carolingian to request help against the powerful Umayyad Emirate of Córdoba in southern Spain, led by Abd ar-Rahman I. The "Spanish Abbasids sought support for their cause in Pepin's Francia; he was content to oblige because the Cordovan dynasty posed a constant military threat to southwestern France". Sulayman al-Arabi the pro-Abbasid Wali (governor) of Barcelona and Girona sent a delegation to Charlemagne in Paderborn, offering his submission, together with the allegiance of Husayn of Zaragoza and Abu Taur of Huesca in return for military aid. The three pro-Abbasid rulers also conveyed that the caliph of Baghdad, Muhammad al-Mahdi, was preparing an invasion force against the Umayyad ruler Abd al-Rhaman I. Following the sealing of this alliance at Paderborn, Charlemagne marched across the Pyrenees in 778 "at the head of all the forces he could muster". His troops were welcomed in Barcelona and Girona by Sulayman al-Arabi. As he moved towards Zaragoza, the troops of Charlemagne were joined by troops led by Sulayman. Husayn of Zaragoza, however, refused to surrender the city, claiming