The Kushans (Soter Megas)
The origins of the Kushans were the group of nomads called by the Chinese "Yueh-chi," that were pushed out of Xinjiang-Mongolia late in the 2nd century BC by the Xiung-nu (probably the "Huns") and moved south and west, eventually displacing the Scythians (Sakas) in Afghanistan.
On their way south, early in the first century A.D., the Kushans had attacked and greatly weakened the Parthian Empire of the Arsacids, which was under pressure from the Rome. Their first conquest was Bactria south of the Oxus, under their ruler, Kujula Kadphises, from where they advanced south, east and west to give them an immense empire covering modern Afghanistan and Pakistan as far east as the Indus and the Punjab and the Ganges Valley as far as Allahabad or Benares.
The chronology is hazy but Kujula seems to have first made himself master of the Kabul Valley, and then annexed Gandhara as far as the Indus, conquering it from the successor of the Parthian, Gondophares. The decisive battle giving him the mastery of what was then northern India was the siege and storm of Taxila some fifteen years later, the victor being either a nameless successor or Kadphises II, known from the coins as Wima Kadphises.*
Wima extended control to the mouth of the Indus, as well as modem Afghanistan, Aria, Sakistan, and Arachosia. Wima was succeeded by the greatest of the Kushan monarchs, the famous Kanishka. Under Kanishka the Kushan empire was extended far into the Ganges Valley, and a capital city of the Indian province established at Mathura. The starting date of his reign is disputed, but may have been around 128 A.D. His northern capital was at Purushapura or Peshawar.
Parthia was now facing the threat of Rome in the west and the Kushans in the east and thus the channels of trade between east and west, long denied by the Parthians, were now unblocked.
The Saka (Scythian) had not been completely displaced by the Kushans; the Western Satraps still ruled in western India, for example. Both were swept aside by the White Huns (Hephthalites) in the 5th century A.D. By then, the more dynamic Sassanians had displaced the Parthians and the empire of the Kushans had shrunk dramatically.
* There is some confusion as to the succession as Vima I Takto (Soter Megas) was probably the father of Vima Kadphises and the grandfather of Kanishka I (the great).