Those northerners actually turned superior Roman tactics -- which they had learned from the Romans as hired mercenaries -- against the Romans. That, and the sheer numbers allowed them to ultimately destroy the empire. Not to mention a whole lot of other shit unconnected with race, like the fact that the Romans had stretched themselves too thin.
all of that is tied into race though, because the romans were stupid poopskins just like modern spaniards and mexicans are. The German Kaiser crushed them easily and created the First Reich.