The Kushan Empire was a relatively short-lived empire that controlled a vast area from the eastern shores of the Caspian Sea across the north and east of modern Iran, Afghanistan, Pakistan and much of the Ganges valley in northern India. From the last years of the first century A.D. to its disintegration in the first half of the 3rd. it controlled all the land trade routes between the Mediterranean, then under the Romans at the height of its power, and China.
The Kushans were an Indo-European people, part of the central Asian Tokhari confederation of tribes, and moved into the then Hellenistic lands of Bactria in northern Iran in the century before Christ. They found themselves up against the Parthians, and turned towards the more lucrative east. By the second half of the 2nd century A.D. they were in India, which was to become their cultural base.
They did not last long. Like the Parthians. they were defeated by the Sassanians, who were to rule most of south-west Asia from the first half of the 3rd century for the next 400 years.
Reference: "Times Atlas of World History"