The Dutch have not always been so peaceful. They indeed decided to attack the Roman Empire!
They really came to Cologne and tried to convince the inhabitants to kick out the Romans and to return to be real Germans.
But the Ubier told them that this is impossible because there had been already a lot of mixed couples and families. But they would stop to support the Empire and support the Germanic idea.
Of course nothing happened. Cologne stood a central heated well fortified Roman town with public water supplies and everything else, what's good and civilised.
Thats obviously the reason, why the Dutch became so peaceful and decided to become independent.