The Indus valley would offer to the military explorers from the West the first taste of the quality of the climate of the India of the plains which they would encounter. The Indus valley in the hot weather would possess little climatic attraction for the Western highlander. Alexander's troops mutinied when they got far beyond the Indus. Any other troops would mutiny under such conditions as governed their outfit and their march. It is more than possible that the great Assyrian conqueror before him encountered much the same difficulty. It is clear, however, historically, that the Assyrian knew and trod the way to Northern Afghanistan (or Baktria), and if we examine the map of Asia with any care we shall see that there is no formidable barrier to the passing of large bodies of people from Nineveh to Herat (Aria), or from Herat to the Indus valley, until we reach the very gates of India on the north-west frontier. Four centuries later than Tiglath Pileser the battle of Arbela was fought to a finish between Alexander and Darius (who possessed both Greek and Indian troops in his army) on a field which is not so very far to the east of Nineveh, and which is probably represented more or less accurately by the modern Persian town of Erbil. The modern town may not be on the exact site of the action, and we know that the ancient town was some sixty miles away from the battlefield. However that may be, we learn that in the general retreat of the Persians which followed the battle, Darius made his way to Ecbatana, the ancient capital of the Medes. There he remained for about a year, but hearing of Alexander's advance from Persepolis in the spring of 330 B.C. he fled to the north-east, with a view to taking refuge with his kinsman Bessos, who was then satrap of Baktria. This gives us the clue to the general line of communication between Northern Mesopotamia and Baktria (or Afghanistan) in ancient days; and the twenty-five centuries which have rolled by since that early period have done little to modify that line.
Until the beginning of the nineteenth century A.D. from the earliest times with which we can come into contact through any human record, this high-road (not the only one, but the chief one) must have been trodden by the feet of thousands of weary pilgrims, captives, emigrants, merchants, or fighting men--an intermittent tide of humanity exceeding in volume any host known to modern days--bringing East into touch with the West to an extent which we can hardly appreciate. It may be said that the straightest road to Baktria did not lie through Ecbatana. It did not; but independently of the fact that Ecbatana was a city of great defensive capacity, and of reasons both political and military which would have impelled Darius to take that route, we shall find if we examine the latest Survey of India map of Western Persia that the geographical distribution of hill and valley make it the easiest, if not the shortest, route. The configuration of Western Persia, like that of Makran and Southern Baluchistan extending to our own north-west frontier, mainly consists of long lines of narrow ridges curving in lines parallel to the coast, rocky and mostly impassable to travellers crossing their difficult ridge and furrow formation transversely, but presenting curiously easy and open roads along the narrow lateral valleys. Ecbatana once stood where the modern Hamadan now stands. The road from Arbil (or Erbil) that carries most traffic follows this trough formation to Kermanshah and then bends north-eastward to Hamadan. From Hamadan to Rhagai and the Caspian gates, which was the route followed by Darius in his flight from Ecbatana, the road was clearly coincident with the present telegraph line to Tehran from Hamadan, which strikes into the great post route eastward to Mashad and Herat, one of the straightest and most uniformly level roads in all Asia. It must always have been so. Remarkable physical changes have occurred in Asia during these twenty-five centuries, but nothing to alter the relative disposition of mountain and plain in this part of Persia, or to change the general character of its ancient highway. All this part of Persia was under the dominion of the Assyrian king when the tribes of Israel left Syria for Armenia. He had but recently traversed the road to India, and he knew the richness of Baktria (of Afghan Turkistan and Badakshan) and could estimate what a colony might become in these eastern fields.