At the beginning of the fourth century B.C., the Greek physician Ktesias, who resided at the court of Artaxerxes II., learnt much from the Persians about India, and was personally acquainted with wise Indians. Little useful information can, however, be derived from the account of India which he wrote after his return in 398 B.C., as it has been very imperfectly preserved, and his reputation for veracity did not stand high among his countrymen. The destruction of the Persian empire by Alexander the Great led to a new invasion of India, which fixes the first absolutely certain date in Indian history. In 327 B.C. Alexander passed over the Hindu Kush with an army of 120,000 infantry and 30,000 cavalry. After taking the town of Pushkalavati (the Greek Peukelaotis) at the confluence of the Kabul and Indus, and subduing the Açvakas (variously called Assakanoi, Aspasioi, Hippasioi, by Greek writers) on the north and the Gandharas on the south of the Kabul River, he crossed the Indus early in 326. At Takshaçila (Greek Taxiles), between the Indus and the Jhelum (Hydaspes), the Greeks for the first time saw Brahman Yogis, or "the wise men of the Indians," as they called them, and were astonished at their asceticism and strange doctrines. Between the Jhelum and the Chenab (Akesines) lay the kingdom of the Pauravas or Pauras, whose prince, called Porus by the Greeks from the name of his people, led out an army of 50,000 infantry, 4000 cavalry, 200 elephants, and 400 chariots to check the advance of the invader. Then on the banks of the Jhelum was fought the great historic battle, in which Alexander, after a severe struggle, finally won the day by superior numbers and force of genius. He continued his victorious march eastwards till he reached the Sutlej (Greek Zadadres). But here his further progress towards the Ganges was arrested by the opposition of his Macedonians, intimidated by the accounts they heard of the great power of the king of the Prasioi (Sanskrit Prachyas, or "Easterns"). Hence, after appointing satraps of the Panjab and of Sindh, he sailed down to the mouths of the Indus and returned to Persia by Gedrosia. Of the writings of those who accompanied Alexander, nothing has been preserved except statements from them in later authors. After Alexander's death the assassination of the old king Porus by Eudemus, the satrap of the Panjab, led to a rebellion in which the Indians cast off the Greek yoke under the leadership of a young adventurer named Chandragupta (the Sandrakottos or Sandrokyptos of the Greeks). Having gained possession of the Indus territory in 317, and dethroned the king of Pataliputra in 315 B.C., he became master of the whole Ganges Valley as well. The Maurya dynasty, which he thus founded, lasted for 137 years (315-178 B.C.). His empire was the largest hitherto known in India, as it embraced the whole territory between the Himalaya and the Vindhya from the mouths of the Ganges to the Indus, including Gujarat.