The part of the Frankish (east) and Saxon (west) Neustria which later became Normandy was not very orientated towards the sea. Living within quiet, unthreatened frontiers, administered by a land state which was not particularly well-organised, this territory became an easy prey when, at the beginning of the 9th century, the Viking threat manifested itself along the Frankish coasts.

As a consequence, Charlemagne organised a system of coastal defences which proved effective at the time of the first reported raid:  that of AD 820 on the lower Seine. But these defenses were soon outflanked and, in AD 841, the city of Rouen was burnt down and important abbeys in the Seine valley were looted or held to ransom. From AD 851, the (mainly Danish) Vikings started to over-winter in camps on several islands in the River Seine. One of these, Jeufosse, became the main launching base for raids all over the neighbouring regions and, particularly, on Paris which was besieged several times.

The Viking presence became almost an annual event and the coastal zones soon became untenable. Along with numerous natives, the monks had to flee from the region, seeking refuge deep in the countryside, carrying the relics, treasures and archives which the Vikings had not plundered. Charles the Bald tried in vain to stem Viking incursions on Paris by building a fortified bridge at Pont-de-l’Arche, near Pîtres, in AD 862. As in eastern Neustria, the Frankish kingdom was also outflanked via western Neustria over many years. This region, under the control of Saxon lords, was also subjected to incessant incursions by Viking forces (mainly Hiberno-Norse from Ireland), particularly in the Bessin and Cotentin areas. Since the defenses could not confront them, in AD 867 Charles the Bald entrusted the Bretons with the task of defending Cotentin and Avranchin. The Bretons were unable to successfully defend these areas.

The Scandinavian colonisation of northern France was under way.