Who made a threat that was responsible for China expanding into the Ordos reigon ?
An early reference to the Xiongnu was by Sima Qian who wrote about the Xiongnu in the " Shiji " ( BCE ) , drawing a distinct line between the settled Huaxia people ( Chinese ) to the pastoral nomads ( Xiongnu ) , characterizing it as two polar groups in the sense of a civilization versus an uncivilized society .