The fifth century marked the collapse of the imperial system in Italy, Germany, Britain, Gaul, Spain, and North Africa, but not of the imperial “idea” …
Imperial Unity and Christian Divisions: The Church AD 450 – 680, John Meyendorff
Ostrogoths, Visigoths, Franks, Burgundians, Vandals: Germanic tribes establishing themselves as rulers over what was the western part of the Roman Empire. While they had the strength to conquer, they had neither the strength nor desire to destroy the cultural legacy. They would accept a legal fiction as foederati (“guests,” or “allies”) of the Romans. But the reality was understood.
The Roman majority would retain Roman law, but the Germanic tribes brought their traditions with them. They also brought their Arian Christianity with them. Eventually, their language and their Arianism would fade away (the Roman bishop being the major agent of this change), but not quickly and often not peacefully on the part of the tribes.