The nomadic Indo-European Yuezhi linked Persian and Chinese civilizations at various periods over five centuries.

Though the earliest known home of the Yuezhi (Yüeh-Chih) people of Central Asia is northern China south of the Gobi Desert (modern Gansu Province), they were apparently, if the images on later Yuezhi coins are accurate, ethnic Caucasians and spoke an Indo-European language called Tokharian by the Greeks. The earliest references to them are in Chinese sources, where they appear as nomads in more or less constant conflict with another nomadic group, the Xiongnu (Hsiung-nu) in the late fifth century b.c.e.
Zhang Qian
Kujūla Kadphises I

Immediately after the last Ice Age, c. 4000 b.c.e., Central Asia had been lush farmland, irrigated by the retreating ice. It continued to be so when Indo-Europeans, the probable ancestors of the Yuezhi, entered the area around 2500 b.c.e. However, in the ensuing millennia, the glacier runoff dried up, leaving vast grasslands less suitable to farming. The Xiongnu finally expelled the Yuezhi from the fiercely contested land around 300 b.c.e., and they settled farther west, establishing a similar nomadic grazing society between the Altai Mountains in western Mongolia and Lake Balkhash in modern Kazakhstan.

Even there they were not safe from the Xiongnu, who found them again a century later and resumed warfare between 176 and 160 b.c.e. The Yuezhi fled to the Oxus Valley in modern Turkmenistan, just north of the Afghan border. This area had been the eastern frontier of Alexander the Great’s Greek expansion of the Persian empire (the Greeks called it Bactria), and here the Yuezhi became the aggressors, conquering the Persian-Greek residents of the Oxus Valley. The Yuezhi maintained their hold on the conquered people by marrying their leaders to the five royal families in the region and adopting Greco-Bactrian culture.

It was this settled Yuezhi pentocracy that the Han Dynasty of China, under Emperor Wu (Wu-ti; 156-87 b.c.e.), contacted in 138 b.c.e. The courtier Zhang Qian was sent out to seek allies to fight against the Xiongnu, who were beginning to raid the Han empire from the north. When he heard the Xiongnu boast of conquering the Yuezhi—they claimed to have a goblet made from the skull of the Yuezhi chief—Zhang Qian felt that revenge might motivate the Yuezhi to join the Chinese against the Xiongnu. He was mistaken. Life was too good in the Oxus Valley, and the Xiongnu were enemies of a past generation.

Nonetheless, almost a century later, the Xiongnu found them again, invading the Oxus Valley in 73 b.c.e. This time, the Yuezhi welcomed an alliance with the Chinese, and with their help repelled the Xiongnu for good in 44 b.c.e.

Sometime around 30 c.e., one of the five Yuezhi kings, Kujūla Kadphises, overpowered his four rivals and created a powerful state that would be the nucleus of a North Indian empire, known to the Chinese as Guishang (Kueh Shen)—the Kushān Dynasty. Not content with ruling the Oxus Valley, Kujūla defeated the Śakas of northern India, expanding into the headwaters of the Ganges and the whole of the Indus valley, and reigning from 30 to 80 c.e. His successor, Vima Takhto, maintained the empire until his death c. 100 c.e., and Vima’s son Vima Kadphises I (r. c. 100-c. 127 c.e.) expanded it into Afghanistan.

The Yuezhi were a significant influence on the development of a number of different cultures. To the Chinese of the Han Dynasty, they represented the first liaison with any group outside of China’s borders. To the Bactrians, they represented a fellow Indo-European culture that began as their conquerors but ended up synthesizing Greco-Bactrian culture with their own. To India, they represented an early empire to the north.

To the later history of the region, the Yuezhi also represent a model for another nomadic group who, a millennium later, would create the largest land empire in history: the Mongols. In fact, horses are a direct historic connection between the Yuezhi contact with China and a major factor in the success of the great khans of the twelfth century. When the Han courtier Zhang Qian visited the Yuezhi in 138 b.c.e., he may have come away without their military aid, but he did not leave empty-handed. He acquired a strategic understanding of the importance of what his report called tien ma, “heavenly horses,” larger by far than any ever seen in China.

These horses must have been Arabians, half again the size of the Asian ponies then in China. Han horse breeders crossed the Chinese pony with these tien ma to produce the hybrid that propelled the armies of the great khans across the steppes of Central Asia, combining the strength and endurance of the Arabian with the agility of the pony. Ironically enough, the battle style of the Mongols—shooting a longbow from a running horse, a skill made possible by the invention of the stirrup—was learned not from the Yuezhi who sold the Han their horses, but from the Xiongnu who used it over the centuries to harass the Yuezhi.

• Christian, David. A History of Russia, Central Asia, and Mongolia: Inner Eurasia from Prehistory to the Mongol Empire. Malden, Mass.: Blackwell, 1999. Section 3, “The Scythic and Hunnic Eras, 1000 b.c.e.-500 c.e.,” includes coverage of the Xiongnu and their interations with the Yuezhi. Although highly academic, this is one of the best sources on this extremely under-studied area and time. Maps.
• Lowe, Michael. Records of Han Administration. New York: Cambridge University Press, 1967. Primary source for the first contact between China and the Yuezhi.
• So, Jenny F., and Emma C. Bunker. Traders and Raiders on China’s Northern Frontier. Seattle: University of Washington Press, 1995. Detailed history of China’s nomadic neighbors throughout history, including the Yuezhi.
• Sun, Alex. The Saga of Zhang Qian. Hong Kong: Orchid Pavilion, 2003. Though a historical novel, this fictional work gives more detail of the daily life of the Yuezhi than any purely historical work available.
• Worden, Robert L., and Andrea Matles Savada, eds. Mongolia: A Country Study. Washington, D.C.: Library of Congress, 1989. U.S. Federal Research Division study on Mongolia; the opening chapter concerns the Yuezhi and their struggle with the Xiongnu.