The Yüeh-chih started westwards from the frontiers of China about 100 B.C. and, driving the Sakas before them, settled in Bactria. Here Kadphises, the chief of one of their tribes, called the Kushans, succeeded in imposing his authority on the others who coalesced into one nation henceforth known by the tribal name. The chronology of the Kushan Empire is one of the vexed questions of Indian history and the dates given below are stated positively only because there is no space for adequate discussion and are given with some scepticism, that is desire for more knowledge founded on facts. Kadphises I (c. 15-45 A.D.) after consolidating his Empire led his armies southwards, conquering Kabul and perhaps Kashmir. His successor Kadphises II (c. 45-78 A.D.) annexed the whole of north-western India, including northern Sind, the Panjab and perhaps Benares. There was a considerable trade between India and the Roman Empire at this period and an embassy was sent to Trajan, apparently by Kanishka (c. 78-123), the successor of Kadphises. This monarch played a part in the later history of Buddhism comparable with that of Asoka in earlier ages[117]. He waged war with the Parthians and Chinese, and his Empire which had its capital at Peshawar included Afghanistan, Bactria, Kashgar, Yarkand, Khotan[118] and Kashmir. These dominions, which perhaps extended as far as Gaya in the east, were retained by his successors Huvishka (123-?140 A.D.) and Vasudeva (?140-178 A.D.), but after this period the Andhra and Kushan dynasties both collapsed as Indian powers, although Kushan kings continued to rule in Kabul. The reasons of their fall are unknown but may be connected with the rise of the Sassanids in Persia. For more than a century the political history of India is a blank and little can be said except that the kingdom of Surashṭra continued to exist under a Saka dynasty.