c.790. Conflict between Offa of Mercia and Charlemagne
From a letter of Alcuin to Colcu (EHD 192), we learn that dissension had lately arisen between Offa of Mercia and Charlemagne, such that each ruler refused landfall to the ships and merchants of the other. Alcuin had heard rumours that he would be sent back to England to help negotiate a peace.
The Acts of the Abbots of Fontenelle (EHD 20), written some forty years later, explain that the cause of the conflict was that Charlemagne had sought Offa's daughter as a wife for his son, and that Offa had replied that this might only be if Offa's son might wed Charlemagne's daughter Bertha. Charlemagne apparently grew furious, and ordered that no English ships be allowed to land on the coast of Gaul, but was restrained by the wise counsel of the abbot of Fontenelle.
Another possible cause of the conflict might be that Charlemagne was harbouring enemies of Offa who had been driven from England. That this did happen we know from letters Charlemagne wrote to the archbishop of Canterbury and to Offa himself (EHD 196 and 197). Ecgberht of Wessex, who was exiled to France in about 789 (see entry), might have sheltered at Charlemagne's court. With the hindsight that comes from our knowledge that Ecgberht conquered all of southern England including Mercia in the 820s, we can see that Ecgberht was potentially Offa's most dangerous foe. If this was at all apparent in the young Ecgberht of 789, Offa might well have hoped that Charlemagne would kill Ecgberht instead of succouring him, and this would be a further reason for the cooling of relations in about 790.
Whatever the reasons behind the breach, it had healed by 796 at the latest, when Charlemagne wrote a very cordial letter to Offa (EHD 197), making provisions that English merchants should be protected by the laws while in Frankish territory, and enjoining that Offa similarly protect Frankish merchants in English territory.