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.
Han–Xiongnu War
In 25 AD, Liu Xiu was established as Emperor Guangwu, restoring the Han throne after a usurpation by the Han official Wang Mang, thus initiating the Eastern Han period. During his reign, the Han empire began to abandon its offensive strategy against the Xiongnu, which allowed the latter to frequently raid the northern frontier. It resulted in large migrations southwards, which led to the depopulation of the frontier regions. During the Eastern Han period, various nomadic peoples were resettled in these frontier regions, serving the Han empire as cavalry against the Xiongnu. With his primary focus still towards the interior of the empire, Emperor Guangwu declined several requests from the western states to reestablish the office of Protector-General of the Western Regions. Early in Emperor Guangwu's reign, King Kang of Yarkand united neighboring kingdoms to resist the Xiongnu. At the same time, he protected the Han officials and people of the former Protector-General, who were still left behind after Wang Mang's reign. In 61 AD, Yarkand was conquered by Khotan and the western states fell in conflict with each other. Taking this opportunity, the Northern Xiongnu recovered their control over the Western Regions, which threatened the security of the Hexi Corridor. In 73 AD, General Dou Gu was sent on a punitive expedition to the Xiongnu and inflicted them a considerable defeat. Immediately, the fertile lands of Hami (Yiwu) was reoccupied and an agricultural garrison established. The next year (74 AD), he expelled the Xiongnu from Turfan and reoccupied the state. The recovery of Hami and Turfan facilitated the reestablishment of the Protector-General, since these important locations were key points to control the Western Regions.