Han dynasty
To the north of China proper, the nomadic Xiongnu chieftain Modu Chanyu (r. 209–174 BC) conquered various tribes inhabiting the eastern portion of the Eurasian Steppe. By the end of his reign, he controlled Manchuria, Mongolia, and the Tarim Basin, subjugating over twenty states east of Samarkand. Emperor Gaozu was troubled about the abundant Han-manufactured iron weapons traded to the Xiongnu along the northern borders, and he established a trade embargo against the group. In retaliation, the Xiongnu invaded what is now Shanxi province, where they defeated the Han forces at Baideng in 200 BC. After negotiations, the "heqin" agreement in 198 BC nominally held the leaders of the Xiongnu and the Han as equal partners in a royal marriage alliance, but the Han were forced to send large amounts of tribute items such as silk clothes, food, and wine to the Xiongnu.
Han–Xiongnu War
The first half of the 1st century BC witnessed several succession crises for the Xiongnu leadership, allowing the Han empire to reaffirm its control over the Western Regions. Huduershi Chanyu was succeeded by his son Punu (蒲奴) in 46 AD, thus breaking the late Huhanye's orders that only a Xiongnu ruler's brother was a valid successor. Bi (比), the Rizhu King of the Right and Huduershi's nephew, was outraged and was declared a rival Chanyu by eight southern Xiongnu tribes in 48 AD. The Xiongnu confederation fell apart in the Northern Xiongnu and Southern Xiongnu, and Bi submitted to the reign of the Han empire in 50 AD. The Han took control of the Southern Xiongnu under Bi, which had 30–40 thousand troops and a population of roughly twice or thrice the size.