But usually, these foreign citizens had developed ties to Rome itself. They were in Latin, they bought into the whole idea of the aristocratic republic. By the 3rd and 4th century CE, though, the empire had been forced to allow the kind of riffraff into their army who didn't really care about the idea of Rome itself. They were only loyal to their commanders. And as you no doubt remember from the historical examples of Caesar, Pompey, Marius, contemporary Afghanistan, this is not a recipe for domestic bliss. So here was Rome, stuck with a bunch of expensive and bloody wars against Germanic peoples who were really good at fighting, and then they had a great idea. Why not fight with these guys?