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.
Xiongnu
The Han dynasty made preparations for war when the Han Emperor Wu dispatched the explorer Zhang Qian to explore the mysterious kingdoms to the west and to form an alliance with the Yuezhi people in order to combat the Xiongnu. During this time Zhang married a Xiongnu wife, who bore him a son, and gained the trust of the Xiongnu leader. While Zhang Qian did not succeed in this mission, his reports of the west provided even greater incentive to counter the Xiongnu hold on westward routes out of China, and the Chinese prepared to mount a large scale attack using the Northern Silk Road to move men and material.