Why were Fankish kings unable to defend their empire against invaders such as the vikings?

Frankish kings struggled to defend their empire against invaders like the Vikings due to a combination of internal divisions and weakened central authority. The feudal system, which relied on local lords for military support, often led to conflicting loyalties and a lack of unified response to external threats. Additionally, the empire faced economic challenges and resource limitations, making it difficult to mobilize a cohesive defense against the swift and brutal Viking raids. These factors culminated in the gradual disintegration of Frankish power and territorial integrity.