The development and consolidation of the Han dynasty in China, particularly in terms of the expansion of population and agriculture, had an impact on the nomadic tribes along its northern border.
In 135 b.c.e. the Han Chinese emperor sent his emissary Zhang Qian (Chang Ch’ien) to find the Yuezhi (Yueh-chih) for a joint offenssive against the Xiongnu (Hsiung-nu). Zhang found the Yuezhi, now calling themselves Kushans, in modern-day Afghanistan but failed to persuade them to renew their emnity against the Xiongnu.
Earlier the Xiongnu had been forced to move west, and they in turn had displaced a confederacy of settled tribes who then came to occupy a wide swath of land across northern India, Bactria, and Gandhara, which are now part of Afghanistan.
These people were known to the Chinese as Yuezhi and to others further west as Indo-Scythians. One branch of the Yuezhi was known as the Kushans, or Kusana, probably as a result of the connection with the Chinese province of Gansu (Kansu).
The Kushans established themselves as the dominant force in the north of the Indian subcontinent, parts of Central Asia, and Afghanistan during the first three centuries of the Common Era. The expansion of the Kushan Empire is most usually ascribed to the leader Kanishka.
The Kushan Empire controlled long stretches of the Silk Road and this served to provide rulers with considerable revenue and power. Under Kaniska, the Kushan Empire was considered an equal of both Rome and China.
Trade goods passed through Kushan territory from both the east and the west, influencing Kushans with trends and ideas from around the world. It was through Kushan territory that Central Asia received early Buddhist ideas and knowledge.
The emperor Kaniska welcomed ideas from other countries and scholars debated Zoroastrianism, Brahmanism, and Greek religious concepts. A lasting memorial to this is seen in the Gandhara school of art. The Kushan culture declined as independent forces in India and, particularly, the Sassanid Empire in Iran began to attain more influence.