One can start with an already tame population of foxes/dogs and select for aggression. Aggression would be a useful trait in a breed desired for protection/guarding.
People have been doing that and yet somehow those aggressive breeds still frequently produce harmless puppies, so it's not obvious that breeders have actually achieved it. Maybe dogs have been domesticated for so long that we can't undo their behavior with just a couple of hundred years. Or maybe we can. It's hard, not obvious.