Contemporary Chinese and Roman sources suggest that, during the 4th century, the Kidarites began to encroach on the territory of Greater Khorasan and the Kushan Empire – migrating through Transoxiana into Bactria, where they were initially vassals of the Kushans and adopted many elements of Kushano-Bactrian culture. The Kidarites also initially put pressure on the Sasanian Empire, but later served as mercenaries in the Sassanian army, under which they fought the Romans in Mesopotamia, led by a chief named Grumbates (fl. 353–358 CE). Some of the Kidarites apparently became a ruling dynasty of the Kushan Empire, leading to the epithet "Little Kushans".