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.
History of the eastern steppe
In 221 BC China was unified by the Qin dynasty and after a brief civil war, more permanently by the Han dynasty (206 BC – 220 AD). About 209 BC the steppe tribes were united by the Xiongnu. The two unifications were connected in that the nomads had to unite to protect themselves from the now-powerful Chinese and the Xiongnu ruler could strengthen himself with Chinese tribute. In the east, the Xiongnu conquered the Xianbei in Manchuria and in the west they drove the Wusun and Yuezhi westward. The Wusun soon disappeared, but the Yuezhi later had a great career as the Kushans of Afghanistan. In China, after about 75 years of internal consolidation and external tribute, Emperor Wu of Han adopted a forward policy. Zhang Qian went west (138-125 BC) and returned with the first good reports of the Western Regions. During the Han-Xiongnu Wars the southern Xiongnu in Inner Mongolia were subjugated and the northern Xiongnu were driven to northern Mongolia where they were broken up around 89 AD. The Han gained control of the Gansu Corridor and the Tarim Basin with some influence westward. This opened the Silk Road and led to the appearance of Chinese silk in the Roman Empire. As the Han declined the Tarim Basin experienced complex shifts of power between the Chinese, Xiongnu remnants and local rulers.
In the 19th century Western imperialism and industrialism began to impact eastern Asia. Steppe nomadism and many traditional Chinese institutions gradually became obsolete, both economically and militarily. In 1860 Russia annexed north and east Manchuria which soon acquired a Russian population. There was a massive migration of Han Chinese into Manchuria and Inner Mongolia which socially removed these areas from the steppe and attached them to China. When the Manchu dynasty fell in 1912 the Mongols claimed that they had been subject to the Manchus rather than China and were therefore now independent. This Russian-backed independence may have prevented Chinese immigration into the area. In recent decades there has been a significant Han migration into Xinjiang.