According to one Roman historian, Varus' mistake was to think that because the German had human shape and human voices, they were humans.
I didn’t think they got very far past the Rhine; unlike England, I don’t think there are many Roman ruins in Germany. Belgium/Holland, maybe; not Germany. Varus lost 10% of the whole Roman army (globally) in that one catastrophic defeat (and it was a couple of years before anyone knew what happened to them).