Xiongnu
The Xiongnu () were a confederation of nomadic peoples who, according to ancient Chinese sources, inhabited the eastern Asian Steppe from the 3rd century BC to the late 1st century AD. Chinese sources report that Modu Chanyu, the supreme leader after 209 BC, founded the Xiongnu Empire.
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.