Final answer:
The Celts and the Anglo-Saxons were frightened of the Vikings due to their violent raids and the inability of the Frankish or Anglo-Saxon armies to defeat them.
Step-by-step explanation:
The Celts and the Anglo-Saxons were frightened of the Vikings due to their violent raids and the inability of the Frankish or Anglo-Saxon armies to defeat them. The Vikings attacked settlements, specifically targeting churches and monasteries for their loot. Their sudden and destructive raids, as well as their reputation for ferocity, instilled fear in the population of Western Europe.