Wusun in the context of "Qilian Mountains"

The Yuezhi were an ancient people first described in Chinese histories as nomadic pastoralists living in an arid grassland area in the western part of the modern Chinese province of Gansu, during the 1st millennium BC. After a major defeat at the hands of the Xiongnu in 176 BC, the Yuezhi split into two groups migrating in different directions: the Greater Yuezhi and Lesser Yuezhi. This started a complex domino effect that radiated in all directions and, in the process, set the course of history for much of Asia for centuries to come.

The Greater Yuezhi initially migrated northwest into the Ili Valley (on the modern borders of China and Kazakhstan), where they reportedly displaced elements of the Sakas. They were driven from the Ili Valley by the Wusun and migrated southward to Sogdia and later settled in Bactria. The Greater Yuezhi have consequently often been identified with peoples mentioned in classical European sources as having overrun the Greco-Bactrian Kingdom, like the Tókharoi and Asii. During the 1st century BC, one of the five major Greater Yuezhi tribes in Bactria, the Kushanas, began to subsume the other tribes and neighbouring peoples. The subsequent Kushan Empire, at its peak in the 3rd century AD, stretched from Turfan in the Tarim Basin in the north to Pataliputra on the Gangetic plain of India in the south. The Kushanas played an important role in the development of trade on the Silk Road and the introduction of Buddhism to China.

The Asii, Osii, Ossii, Asoi, Asioi, Asini or Aseni were an ancient Indo-European people of Central Asia, during the 2nd and 1st centuries BCE. Known only from Classical Greek and Roman sources, they were one of the peoples held to be responsible for the downfall of the Greco-Bactrian Kingdom. In Greek Mythology they were the children of Iapetus and Asia.

Modern scholars have attempted to identify the Asii with other peoples known from European and Chinese sources including the: Yuezhi, Tocharians, Issedones/Wusun and/or Alans.