Xiongnu
Sinologist Edwin Pulleyblank argued that the Xiongnu were part of a Xirong group called Yiqu, who had lived in Shaanbei and had been influenced by China for centuries, before they were driven out by the Qin dynasty. Qin's campaign against the Xiongnu expanded Qin's territory at the expense of the Xiongnu. In 215 BC, Qin Shi Huang sent General Meng Tian to conquer the Xiongnu and drive them from the Ordos Loop, which he did later that year. After the catastrophic defeat at the hands of Meng Tian, the Xiongnu leader Touman was forced to flee far into the Mongolian Plateau. The Qin empire became a threat to the Xiongnu, which ultimately led to the reorganization of the many tribes into a confederacy.In 209 BC, three years before the founding of Han China, the Xiongnu were brought together in a powerful confederation under a new chanyu, Modu Chanyu. This new political unity transformed them into a more formidable state by enabling formation of larger armies and the ability to exercise better strategic coordination. The Xiongnu adopted many of the Chinese agriculture techniques such as slave labor for heavy labor, wore silk like the Chinese, and lived in Chinese-style homes. The reason for creating the confederation remains unclear. Suggestions include the need for a stronger state to deal with the Qin unification of China that resulted in a loss of the Ordos region at the hands of Meng Tian or the political crisis that overtook the Xiongnu in 215 BC when Qin armies evicted them from their pastures on the Yellow River.
Han–Xiongnu War
In 119 BC, when the Xiongnu suffered a catastrophic defeat by the Han armies, the Chanyu moved his court (located in present-day Inner Mongolia) to another location north. This had the desired result that the Xiongnu were separated the Wuhuan people, which also prevented the Xiongnu from exacting many resources from the Wuhuan. The Han court placed the Wuhuan in tributary protection and resettled them in five northeastern commandaries, namely Shanggu, Yuyang, Youbeiping (present-day Hebei), Liaoxi, and Liaodong (present-day Liaoning). A new office, the Colonel-Protector of the Wuhuan, was established (near present-day Beijing) in order to prevent contact between the Wuhuan with the Xiongnu and to use them to monitor the Xiongnu activities. Nevertheless, the effective Han control over the Wuhuan was lacking through much of the Western Han period, since the Xiongnu had considerable military and political influence over the Wuhuan while relations between the Wuhuan and Han often remained strained at best. This can be exemplified by a situation in 78 BC, when the Xiongnu led a punitive campaign against the Wuhuan, resulting in General Fan Mingyou (范明友) leading a Han army to impede further incursions. When they learned that the Xiongnu had left by the time the army arrived, the Han court ordered Fan to attack the Wuhuan instead, killing 6000 Wuhuan men and three chieftains, since the Wuhuan had recently raided Han territory. Only in 49 AD, when 922 Wuhuan chieftains submitted during Emperor Guangwu's reign, did many of the Wuhuan tribes come under tributary system of the Han empire. The Han court provided for the Wuhuan and in return the Wuhuan tribes guarded the Han frontier against the Xiongnu and other nomadic peoples.