Final answer:
Charlemagne's empire falling to other Germanic tribes led to the fragmentation of the empire into three territories and the subsequent rise of feudal society in medieval Europe.
Step-by-step explanation:
The event caused by Charlemagne's empire falling to other invading Germanic tribes in western Europe was the fragmentation of the empire into three territories: the Kingdom of the West Franks, the Kingdom of the East Franks, and a middle region called Lotharingia. This fragmentation weakened the stability of the kingdoms and led to constant conflict as each subsequent generation of rulers faced diminished holdings and the need to wage war for more land. The collapse of Charlemagne's empire ultimately paved the way for the rise of feudal society in medieval Europe.