Complex sentence: Just as the Xiongnu expanded southward into Yuezhi territory around 160 BCE under their leader Modun, it is thought they also occupied the Ordos area during the same period, when they came in direct contact with the Chinese.
Split into simpler sentences: Just as the Xiongnu expanded southward into Yuezhi territory around 160 BCE under their leader Modun. It is thought the Xiongnu also occupied the Ordos area during the same period, when they came in direct contact with the Chinese.