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.
Han dynasty
The Eastern Han (), also known as the Later Han (), formally began on 5 August 25, when Liu Xiu became Emperor Guangwu of Han. During the widespread rebellion against Wang Mang, the state of Goguryeo was free to raid Han's Korean commanderies; Han did not reaffirm its control over the region until AD 30. The Trưng Sisters of Vietnam rebelled against Han in AD 40. Their rebellion was crushed by Han general Ma Yuan (d. AD 49) in a campaign from AD 42–43. Wang Mang renewed hostilities against the Xiongnu, who were estranged from Han until their leader Bi (比), a rival claimant to the throne against his cousin Punu (蒲奴), submitted to Han as a tributary vassal in AD 50. This created two rival Xiongnu states: the Southern Xiongnu led by Bi, an ally of Han, and the Northern Xiongnu led by Punu, an enemy of Han.