If you are talking about the inroads of the Angles and Saxons into Britain, that is much later, and their success hinged on Roman withdrawal for reasons completely unconnected with their raiding.
First of all the Teutones were but one tribe.
Second, as with "American Indians", there is a real question whether there were any "Germans" before the Romans identified them as a "group", and Tacitus later idealized them in his Germania, which is as much about what he found unsatisfactory in the Romans of his day as it is about the "noble savages" he so eruditely anthropologized.
You might also investigate why the East Romans at one point made wearing trousers unlawful.
For the "Romanization" of Germans, a good start is Augsburg.
Best regards. S&W R.I.P.