During the Sassanid era, the activities of three main powers in the east of Iran made serious challenges for the Sassanid government. The Kushan Kingdom conquered the entire territories of Yuezhian and Tocharian and took the control of a large land between Seyhun and Ceyhun. The most powerful Kushani king, Kanishka, promulgated Buddhism and until the entrance of Islam to this land, Buddhism was the dominant religion in this territory. The Hephthalites, or the White Huns, were nomadic tribes who intruded on Iranian northwestern borders since the mid-4th century A.D. The Hephthalites showed great resistance against the Sassanids. However, in the mid-6th century, at the time of Khosrow I's reign, the Sassanids defeated the Hephthalites. The threatening presence of Turk tribes in Mesopotamia began around the fifth century A.D. The Turks migrated to the western lands in great populations at the time of Khosrow Anushirvan's reign. After the fall of the Hephthalites, they not only came to political power but also obtained great economic power since the longest Caravan route that connected the west to the east was controlled by them. The Turks were ultimately defeated by Bahram Chobin during the Sassanid Empire era and the reign of Hormizd IV.
