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.