Germanics were the main character in the disintegration of the Roman Empire. Especially important was the event in 476 when Odoacer, a Germanic military chief, defeated the last Roman emperor, Romul Augustus.
All the Occidental part of the Roman Empire was divided into small Germanic kingdoms, despite there wasn't many population. So, the Germanic power wasn't caused for the number of soldiers and citizens they were, but it was the suitable substitute for the political and military structure of the empire defeated. Also the Germanics converted to Christianity.
Anyway, those kingdoms supported themselves in basic barbarian elements. One of the most important was the ban, power that each king had to administrate, make laws and orders, for example. So, the Germanic kings had a huge power. However, that power wasn't enough.
