Francia, the land of the Franks (roughly corresponding to most of modern France, Belgium, The Netherlands, Luxembourg and western Germany) was first raided by the vikings in 799. The emperor Charlemagne reacted vigorously, creating coastguard units and stationing fleets on major rivers. Except for the exposed coast of Frisa, Francia experienced no serious Viking raids until the 830s when the civil wars between the emperor Louis the Pious and his sons had begun to undermine royal authority and with it the effectiveness of the coastal defences. In 842 the Vikings established a permanent base on the island of Noirmoutier, near the mouth of the River Loire, and thereafter were a permanent presence on the Frankish territory for the next seventy years.
The main areas of Viking activity in Francia were the valleys of the rivers Seine and Loire, Flanders, Frisia and the Rhineland, all areas where navigable rivers offered the Vikings easy routes inland. Preoccupied by dynastic conflicts, Francia's Carolingian rulers were rarely able to concentrate on dealing with the Viking threat and were often reduced to buying them off with tribute. Towns were left exposed to attack because rulers such as Charles the Blad were reluctant to allow them to build defences in case they should be turned into strongholds by rebel princes and nobles. For some of these rebels, such as Pippin II of Aquitaine, the Vikings were event welcome as allies. Viking raids reached their peak in the period 879-92 when the Rhineland, the Ardennes, Flanders and the Seine valley were systematically ravaged. But the Franks were finally getting the measure of the Vikings. Able warrior kings such as Odo of West Francia (France) and Arnulf of East Francia (Germany) actively sought to bring the Vikings to battle and built fortresses and town walls across the whole of the region between the Seine and the Rhine. Faced with resistance wherever they went, and a severe famine in the winter of 891-2, the main Viking force withdrew to England in 892 where it fared no better. The worst of the Viking Age in Francia was now over but there remained Viking armies on the Seine and the Loire. The integration of the Seine Vikings into Frankish society was begun when their leader, Rollo, was made count of Rouen by Charles the Simple in 911. Normandy, as Rollo's territories came to be known, created problems of its own, but at least these were the familiar ones of dynastic and territorial ambitions that kings faced from any over-mighty subject. the problems of the Vikings on the Loire was solved when the Bretons captured their base at Nantes in 937. The last Viking raids on Francia occurred in the early 11th century.
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames & Hudson 2000. pp 75-76.