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.
Yuezhi
Gradually the Xiongnu grew stronger and war broke out between them and the Yuezhi. There were at least four wars between the Yuezhi and Xiongnu according to the Chinese accounts. The first war broke out during the reign of the Xiongnu monarch Touman (who died in 209 B.C) who suddenly attacked the Yuezhi. The Yuezhi wanted to kill Modu, the son of the Xiongnu king Touman kept as a hostage to them, but Modu stole a good horse from them and managed to escape to his country. He subsequently killed his father and became ruler of the Xiongnu. It appears that the Xiongnu did not defeat the Yuezhi in this first war. The second war took place in the 7th year of Modu era (203 B.C.). From this war, a large area of the territory originally belonging to the Yuezhi was seized by the Xiongnu and the hegemony of the Yuezhi started to shake. The third war probably was at 176 BC (or shortly earlier) and the Yuezhi were badly defeated.