Leaving, therefore, the direct road to Babylon by the line of the Euphrates undefended, he selected a position which possessed all the advantages of the Mesopotamian plain, being open, level, fertile, and well supplied with water, while its vicinity to the eastern and northern provinces, made it convenient for a rendezvous.
This position was on the left or east bank of the Tigris, in the heart of the ancient Assyria, not more than thirty miles from the site of Nineveh.

Here, in the region called by the Greeks Adiabene, extended between the Tigris and the river Zab or Lycus, a vast plain broken by scarcely any elevations, and wholly bare of both shrubs and trees.

The few natural inequalities which presented themselves were levelled by order of Darius, who made the entire plain in his front practicable not only for cavalry but for chariots.

At the same time he planted, in the places where Alexander's cavalry was likely to charge, spiked balls to damage the feet of the horses.
Meanwhile, Alexander had quitted Egypt, and after delaying some months in Syria while his preparations were being completed, had crossed the Euphrates at Thapsacus and marched through northern Mesopotamia along the southern flank of the Mons Masius, a district in which provisions, water, and forage were abundant, to the Tigris, which he must have reached in about lat.

36 deg.