Ordos Plateau
In Chinese accounts, the Xiongnu first appear at Ordos in the "Yizhoushu" and "Shanhaijing" during the Warring States period before it was occupied by Qin and Zhao. It is generally thought to be their homeland, however when exactly they came to occupy the region is unclear, and it might have been much earlier than traditionally thought, as suggested by archaeological finds. As the Xiongnu expanded southward into Yuezhi territory around 160 BC under their leader Modun, the Yuezhi in turn defeated the Sakas (Scythians) and pushed them away at the Issyk Kul. It is thought the Xiongnu also occupied the Ordos area during the same period, when they came in direct contact with the Chinese. From there, the Xiongnu conducted numerous devastating raids into Chinese territory (167, 158, 142, 129 BC). The Han Dynasty started to fight the Xiongnu in the 2nd century BC under emperor Han Wudi, and colonized the area of the Ordos under Shuofang commandery in 127 BC. Prior to the campaign, there were already commanderies established earlier by Qin and Zhao until they were overrun by the Xiongnu in 209 BC.
History of the Han dynasty
The Eastern-Han court colonized and periodically reasserted the Chinese military presence in the Western Regions only as a means to combat the Northern Xiongnu. Han forces were expelled from the Western Regions first by the Xiongnu between 77–90 CE and then by the Qiang between 107–122 CE. In both of these periods, the financial burdens of reestablishing and expanding western colonies, as well as the liability of sending financial aid requested by Tarim-Basin tributary states, were viewed by the court as reasons to forestall the reopening of foreign relations in the region.