Two areas of the Carolingian empire came under attack are Frisia, and Aquitaine. In Frisia the Vikings' principal targets were trading centres, particularly the prosperous market of Dorestad, which was first sacked in 834. The Anglo-Saxon Chronicle provides nearly all the information about the Viking raids. From 876 to 911, the Scandinavians capitalised on the position of strength to colonise areas of England and Francia. In 876 the Anglo-Saxon Chronicle contained the significant report: 'In this year Halfdan shared out the lands of the Northumbrians, and they proceeded to plough and to support themselves'. Scandinavian chiefs ruled large tracts of Frisia for long periods without any apparent attempt by their followers to colonise the region, and until the mid-870s the armies which occupied Frankish or Anglo-Saxon territory over many years likewise gave no indication of wishing to take political control. The raids were motivated by militant paganism, there is equally little support for this in contemporary sources.

Louis the Pious, however, after the death of Pippin in 838, tried to confine Louis the German once again to Bavaria (839) in order to promote the interests of Charles. It was from Bavaria that the East Frankish kingdom was created. The Carolingian brothers' mutual hostility encouraged the Vikings to redouble their attacks on the Frankish kingdoms, which affected especially Lothar's territory. Even after 843, Bavaria still remained Louis' most important power base. When Lothar I died in 856 his Middle Kingdom was divided among his sons. When Lothar II died in 869, Charles II immediately invaded the Middle Kingdom while his brother was detained at Regensburg. The inheritance of Lotharingia altered the demands on the East Frankish king, for now he had to beat back the Vikings. For the first time the western frontier of Lotharingia appeared as the frontier of the East Frankish kingdom; the Treaty of Ribemont (880) sealed the agreement.