DURING the century following the death of Asoka, interest centres on the north-western threshold of India, as it had done in Alexander's time nearly a hundred years before. The Mauryas grow weak and soon their rule is confined to Magadha. They vegetate until, about 185, a "mayor of the palace", as Sylvain Lévi called him, Pushyamitra, sets up in their place his own family, known to history as the Sungas.
The decisive events which now took place in the west had their beginnings in the time of Asoka, about 250. The Seleucid Empire, ruled by Antiochos III (261-246), lost two provinces, Parthia and Bactriana, which emancipated themselves simultaneously. The Parthians, whom the Indians called Pahlavas, were related to the nomads of the Turkoman steppes and occupied the country south-east of the Caspian. The Bactrians bordered on the Parthians on the north-east and were settled between the Hindu Kush and the Oxus; the number and wealth of their towns were legendary. These two peoples seem to have taken advantage of the difficulties of Antiochos and his successors, Seleucos II (246-226) and III (226-223) in the west to break away. The Parthian revolt was a national movement, led by Arsaces, the founder of a dynasty which was to rule Persia for nearly five hundred years. The Bactrian rising was brought about by the ambition of a Greek satrap, Diodotos, and represents an outbreak of Hellenism in the heart of Asia. There is no doubt that the formation of these enterprising nations on the Indo-Iranian border helped to shake the empire of Asoka in the time of his successors. One result, at any rate, was that India was conquered by Hellenism more effectively than in Alexander's day. This unexpected consequence is explained by the hold which Greek colonization had in Bactriana. We remember with what determination Alexander strove to take complete possession of that country, establishing garrisons and founding cities. Short-lived as were the victories gained by the conqueror, this part of his work had not been fruitless.
The Punjab, once a Persian satrapy and then a province of Alexander, was to find itself still more exposed to attack, now that smaller but turbulent states had arisen at its doors. After Diodotos I and II, the King of Bactriana was Euthydemos, who went to war with Antiochos the Great of Syria. Peace was concluded with the recognition of Bactrian independence about 208. But during hostilities Syrian troops had crossed the Hindu Kush and, entering the Kabul valley, had severely despoiled the ruler, Subhagasena. Demetrios, the son of Euthydemos, increased his dominions not only in the present Afghanistan but in India proper, and bore the title of King of the Indians (200-190). Between 190 and 180 there were Greek adventurers reigning at Taxila, named Pantaleon and Agathocles. From 160 to 140, roughly, Kabul and the Punjab were held by a pure Greek, Milinda or Menander, who left a name in the history of Buddhism. About 155 he conquered the whole of the lower Indus and Kathiawar, waged war in Rajputana and Oudh, took Mathura (Muttra) on the Jumna, and even reached Pataliputra. He was severely defeated by Pushyamitra, who was the chief defender of the Hindu world, although the termination of his name in mitra has caused some to suppose that he was of Iranian origin. It should be added that both in policy and in religion or literature this first Sunga stands for a definitely Indian reaction against every outside influence, and particularly for a Brahman opposition to Buddhism, which with its application to mankind in general had so strongly appealed to the greatest of the Mauryas.
Greek intercourse with India worked both ways. A Greek named Heliodoros, who was sent on a mission to Besnagar, near Bhilsa in Central India, by Antialciadas, King of Taxila, set up a column in honour of Vishnu Vasudeva, declaring himself his follower (bhāgavata). The monolithic pillar and inscription still survive, and their evidence agrees with that of plentiful coins to prove what interpenetration there was between Greeks and Indians at this time.
Bactriana was, at least in the north, a barrier between Parthia and India. India was therefore less exposed to attack from Parthia, Nevertheless, there was at least one Parthian ruler, Mithradates I (171-136) who annexed the country of Taxila for a few years, about 138. Indo-Parthians, like Indo-Greeks, are attested by coins even after the event which was to put an end to the independence of both Parthia and Bactria.
That event was a new invasion, resulting from a movement of tribes which had taken place far away from India, in the Mongolian steppes. About 170 a horde of nomadic Scythians, the Yueh-chi or Tokharians, being driven from Gobi, the present Kansu, by the Hiung-nu or Huns, started on a wild migration which upset the whole balance of Asia. They fell on the Sakas, who were Iranianized Scythians dwelling north of the Persian Empire, and settled in their grazing-grounds north of the Jaxartes (Sir Darya). The expelled Sakas fell on Parthia and Bactriana, obliterating the last vestiges of Greek rule, between 140 and 120. Then the Tokharians, being defeated in their turn by the Wu-sun tribe, established themselves on the Oxus, and after that took all the country of the Sakas in eastern Iran, at the entrance to India. That entrance was forced in the first century after Christ. It was the last Indo-European invasion of ancient India, for the Tokharians and Sakas were two offshoots of the Scythian branch, the most easterly branch of the Indo-European stock.
The conquest of India was the work of the Kushans (Kushāṇa), a dynasty which united the Yue-chi tribes and established their dominion both over their own kinsfolk, the Sakas of Parthia, and over the peoples of the Punjab. At this point great difficulties in dating arise. The accession of the principal king of this line, Kanishka, was placed at uncertain dates between 57 B.C. and A.D. 200. The excavations of Sir John Marshall at the site of Taxila have made it possible to reduce the range to about the end of the first century of our era. Chinese history also supplies some information. Communications between China and the valley of the Oxus were easy in the time of the Emperor Wu-ti, to whom the traveller, Chang-kien brought information about the West (120 B.C.), but they were suddenly interrupted at the beginning of our era, and were only restored by the victorious campaigns of Pan-ch'ao (73-102) against the Kushans. Kanishka seems to have become king after this defeat of the Yueh-chi, who had become an Indian power. Accordingly Vincent Smith, after first adopting A.D. 78, which appeared the most probable, finally chose A.D. 120, and we may agree with him that this date marks the beginning of the "Saka" period inaugurated by Kanishka.1
The order in which the chief Kushan kings followed is still doubtful.2 It is generally agreed that Kanishka came after Kadphises I (Kujula Kara Kadphises) and II (Vima Kadphises). The former of these two, a Bactrianized Scythian, must, in Dr. Smith's view, have assumed power about A.D. 40. He seized Gandhara and the country of Taxila from Gondophares, the Parthian prince who, according to the apocryphal Acts of the Apostles, received St. Thomas. His son Vima (78-110) carved out a great empire for himself, embracing the Punjab and the whole western half of the Ganges basin. This seems to have been the empire the northern parts of which bore the shock of the Chinese armies led by Pan-ch'ao. Vima ventured to seek the hand of a princess of the Chinese court in 90, and the Chinese general, who had marched his troops as far as the Caspian, kept his envoy prisoner. The Kushan king, taking offence, sent 70,000 horsemen over the Pamir, but that enormous host was annihilated in descending on Kashgar or Yarkand. But this defeat, although it gave the Emperor Ho-ti Indian subjects, does not seem to have weakened the prestige of the Kushans inside India. In order to avoid the risk of error in this account, we should remember that it may have been Kanishka, not Kadphises II, who came into conflict with China, and that the Saka era may begin, not in 120 with Kanishka, but in 78 with Kadphises II.
But let us go on with our story, even if it is hypothetical. There seems to have been an interval of about ten years between Kadphises and Kanishka (? 120-? 162). The latter was the son of one Vajheshka and no relation of his predecessor; he seems to have come from Khotan, not Bactriana, and indeed he spent the summer at Kapisi in Paropanisadæ and the winter at Purushapura (Peshawar), The axis of his empire was no longer in the middle of the Græco-Iranian country.
His warlike activity was displayed chiefly in the northern districts. He conquered Kashmir. He established his rule over the Parthians beyond the Pamir, in the Serindian regions where Kadphises II had been defeated, and it was now Chinese influence that retreated in those parts. It is a thing to be noted, that whenever a power extending to Iran or Serindia predominates in India, there is a recrudescence of Buddhism, and when a purely Indian dynasty comes to the fore there is usually a Brahmanic reaction. The reign of Kanishka coincides with a very great development of Buddhist propaganda.
Like Asoka, Kanishka called the Buddhists together in a council, but it was held in the recently annexed Kashmir. It was instructed to draw up definitive commentaries on the Canon and to engrave them on bronze. This laying down of dogma is an important fact in the history of the faith; a neo-Buddhism appears, in which the metaphysical element tends to prevail over the moral, and at the same time foreign factors are blended with Indian.
If we would see for ourselves, clearly and beyond possibility of doubt, the many elements which combined and conflicted in the Kushan Empire, we have only to examine the abundant coins of the period. The Persian title, " King of Kings," is found together with the Greek 
the Indian Adhirāja, and even Devaputra, the Sanskrit translation of the Chinese notion, "Son of Heaven." Coins bear the figures of Mithra, Siva, Buddha, and Heracles indiscriminately. The new Buddhism, which, in contrast to the old, was to be called the "Great Vehicle", grew up in a world scored across by the different influences which these names reveal.
The age of Kanishka was one of prosperity and magnificence. The great Asiatic routes which crossed the Kushan Empire made it wealthy. The inheritance of the Græco-Buddhist art of Gandhara was adapted to express native ideals. An extraordinary personality, Asvaghosha, Buddhist teacher and philosopher, poet and musician, opened many paths to the sacred and profane literature of later ages. The metaphysician Nagarjuna and the physician Charaka inaugurated great traditions.
But it was the destiny of India that no large part of the country should be united for more than two or three reigns, so that its culture could never be kept at the same level of brilliance for more than a few generations. The successors of Kanishka, like those of Asoka, almost at once allowed the Empire to fall into jeopardy. Of his two sons, Vasishka and Huvishka, who had shared the power with him, only the second survived him (? 162-? 182). We know little more of him than his name, or of Vasudeva I. The power of the Kushans in the third century was reduced to Bactriana, with Kabul and Gandhara, and they fell beneath the yoke of the Sassanids.
1 We should note that the Saka era began a hundred or a hundred and fifty years after the destruction of the Saka state of Parthia, and that it was the Yueh-chi who had destroyed that state. Saka dynasties continued to reign in a certain independence from the lower Indus to Kathiawar until the fifth century.
2 A. A. Macdonell, India's Past, 1927, p. 265: (Kanishka) "may possibly have preceded the two Kadphises kings; in that ease his date ... would be a.d. 78."