Is Persia a Kingdom or an Empire?

Greece isn't like Persia. It's not a kingdom or an empire, it's a whole bunch of little independent city states that come together to form these leagues and they do not have their business together in Greece at all. So this lingering question as Persia gets into its golden age after the Greek wars is, well, Greece settle down or is that whole Greece thing going to come back to bite the rest of the world? I guess we're fixing to find out.