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
As the Xiongnu economic and military situation deteriorated, the Xiongnu were willing to renew peace during the reigns of Huyandi Chanyu (r. 85-69 BC) and Xulüquanqu Chanyu (r. 68-60 BC), but the Han court gave only one option, tributary submission. After Xulüquanqu Chanyu's death in 60 BC, a Xiongnu civil war broke loose in 57 BC over the succession, which fully fragmented the Xiongnu confederation with many contenders. In the end, only Zhizhi Chanyu and Huhanye Chanyu survived the struggle to power. After Zhizhi Chanyu (r. 56–36 BC) had inflicted serious losses against his rival Huhanye Chanyu (r. 58–31 BC), Huhanye and his supporters debated whether to request military protection and become a Han vassal. In 53 BC, Huhanye decided to do so and surrendered to the reign of the Han empire.