The external history of India may be considered to begin with the Greek invasion in 327 B.C. Some indirect trade between India and the Levant seems to have existed from very ancient times. Homer was acquainted with tin and other articles of Indian merchandise by their Sanskrit names; and a long list has been made of Indian products mentioned in the Bible. In the time of Darius (see PERSIA) the valley of the Indus was a Persian satrapy. But the first Greek historian who speaks clearly of India was Hecataeus of Miletus (549-486 B.C.); the knowledge of Herodotus (450 B.C.) ended at the Indus; and Ctesias, the physician (401 B.C.), brought back from his residence in Persia only a few facts about the products of India, its dyes and fabrics, its monkeys and parrots. India to the east of the Indus was first made known in Europe by the historians and men of science who accompanied Alexander the Great in 327 B.C. Their narratives, although now lost, are condensed in Strabo, Pliny and Arrian. Soon afterwards Megasthenes, as Greek ambassador resident at a court in Bengal (306-298 B.C.), had opportunities for the closest observation. The knowledge of the Greeks and Romans concerning India practically dates from his researches, 300 B.C. Alexander's march. Alexander the Great entered India early in 327 B.C. Crossing the lofty Khawak and Kaoshan passes of the Hindu Kush, he advanced by Alexandria, a city previously founded in the Koh-i-Daman, and Nicaea, another city to the west of Jalalabad, on the road from Kabul to India. Thence he turned eastwards through the Kunar valley and Bajour, and crossed the Gouraios (Panjkora) river. Here he laid siege to Mount Aornos, which is identified by some authorities with the modern Mahaban, though this identification was rejected by Dr Stein after an exhaustive survey of Mount Mahaban in 1904. Alexander crossed the Indus at Ohind, 16 m. above Attock, receiving there the submission of the great city of Taxila, which is now represented by miles of ruins near the modern Rawalpindi. Crossing the Hydaspes (Jhelum) he defeated Porus in a great battle, and crossing the Acesines (Chenab) near the foot of the hills and the Hydraotes (Ravi), reached the Hyphasis (Beas). Here he was obliged by the temper of his army to retrace his steps, and retreat to the Jhelum, whence he sailed down the river to its confluence with the Indus, and thence to Patala, probably the modern Hyderabad. From Patala the admiral Nearchos was to sail round the coast to the Euphrates, while Alexander himself marched through the wilds of Gedrosia, or modern Makran. Ultimately, after suffering agonies of thirst in the desert, the army made its way back to the coast at the modern harbour of Pasin, whence the return to Susa in Persia was comparatively easy.