Charlemagne’s letter to Offa of Mercia is the oldest surviving diplomatic document in English History. The Franks and Offa’s Mercian kingdom had endured a mixed past. From promising signs and trading agreements they had then clashed. Prior to the letter from Charlemagne to Offa there had been plots to overthrow the Pope, possibly instigated by Offa. Charlemagne had offered the marriage of one of his daughters into the Mercian Royal Family, countered by Offa asking that his son married into the Frankish Court. This sparked outrage that led to shipping from England being barred from ports under Charlemagne’s control: most of Western Europe.
Charlemagne’s letter to Offa followed the Council of Frankfurt, at which the English Church had been approached by the Franks for support. This council paved the way for diplomatic ties to be reopened and led to the letter that Charlemagne wrote. The letter itself is primarily about the rights of pilgrims to safe, tariff-free, passage. It touches upon Trade though.
Charles, by the grace of God king of the Franks and Lombards, patrician of the Romans, to his esteemed and dearest brother Offa, king of the Mercians, sendeth health; First, we give thanks to God Almighty for the purity of the Catholic faith, which we find laudably expressed in your letters. Concerning pilgrims, who for the love of God or for the salvation of their souls wish to visit the residence of the holy apostles, let them go peaceably without any molestation; but if persons not seeking the cause of religion but that of gain be found among them, let them pay the customary tolls in proper places. We will, too, that traders have due protection within our kingdom, according to our mandate, and if in any place they suffer wrongful oppression, let them appeal to us or our judges, and we will see full justice done. Let your grace also be apprised that we have sent some token of our regard, out of our dalmatics and palls, to each episcopal see of your kingdom and of Ethelredās, as an almsgiving, for the sake of our apostolical lord Adrian (the late pope) earnestly begging that you would order him to be prayed for, not as doubting that his blessed soul is at rest, but to show our esteem and regard to our dearest friend. Moreover we have sent somewhat out of the treasures of those earthly riches which the Lord Jesus has granted to us of his unmerited bounty, for the metropolitan cities; and for yourself a belt, an Hungarian sword, and two silk cloaks.
Source: Quoted on various websites, originates from: Edward P. Cheyney, Readings in English History, Boston: Ginn and Company, 1922, p. 56