you probably know that the word french comes from frank, and that the franks were a german-speaking people that invaded the empire in the year 406, in the event known as the "crossing of the rhine".

but, much like the surprise attack in 367, the franks were not a "tribe" so much as they were a confederacy. 

historians today do not think that the franks were an ethnic group at all. rather, frank is a german word that translates to free.

it seems as though it was, rather, a confederacy of "freemen" bent on abolishing the empire once and for all.

this is a theme throughout the roman occupation of western europe, and he gets the point across - rome could only hold these lands via massive occupation forces, and they would consistently revolt the moment the romans left.