The first Germanic tribes, believed to have come from Scandinavia.
The Saxons (combined eastern Germanic tribes) remained largely independent in north and east Germany from where they together with the Scandinavian Jutes and Angles also moved into Roman Britain and where they became the rulers after the collapse of the Empire.The Frisii also joined in but they stayed largely in their home land, the coastal regions in the north of the Low Country.While there certainly was a lot of violence involved, evidence indicates that in most situations the newcomers (often less than 10% of the total local population, sometimes as low as 2%) rapidly integrated with the local population.This in particular was the case where the newcomers settled in Celtic lands; they very rapidly replaced the local culture and language.In order to manage the continuous pressure of these people on their boarders, the Romans allowed some of them to settle in foederati within the Empire.
Bound by treaties they had to provide military services to the Empire.
This was also the case in later migrations and invasions.
Within the tribal societies there was a very strong sense of local power and the various migrating tribes and sub tribes immediately established their tribal authority in the areas they concurred.
Frankenland in modern Germany is an example of a rather rapid transformation.
This and at the same marks the furthest corner of where the Franks migrated too.
The Eastern Germanic tribes formed the confederation of the Goths they were pushed along towards the river Danube and eventually settled in Italy and Spain.