The main strength of his cavalry consisted in two chosen regiments of cuirassiers, one Macedonian and one Thessalian, each of which was about fifteen hundred strong. They were provided with long lances and heavy swords, and horse as well as man was fully equipped with defensive armor. Other regiments of regular cavalry were less heavily armed, and there were several bodies of light-horsemen, whom Alexander's conquests in Egypt and Syria had enabled him to mount superbly.
A little before the end of August, Alexander crossed the Euphrates at Thapsacus, a small corps of Persian cavalry under Mazaeus retiring before him. Alexander was too prudent to march down through the Mesopotamian deserts, and continued to advance eastward with the intention of passing the Tigris, and then, if he was unable to find Darius and bring him to action, of marching southward on the left side of that river along the skirts of a mountainous district where his men would suffer less from heat and thirst, and where provisions would be more abundant.
Darius, finding that his adversary was not to be enticed into the march through Mesopotamia against his capital, determined to remain on the battle-ground, which he had chosen on the left of the Tigris; where, if his enemy met a defeat or a check, the destruction of the invaders would be certain with two such rivers as the Euphrates and the Tigris in their rear.
The Persian King availed himself to the utmost of every advantage in his power. He caused a large space of ground to be carefully levelled for the operation of his scythe-armed chariots; and he deposited his military stores in the strong town of Arbela, about twenty miles in his rear. The rhetoricians of after-ages have loved to describe Darius Codomanus as a second Xerxes in ostentation and imbecility; but a fair examination of his generalship in this his last campaign shows that he was worthy of bearing the same name as his great predecessor, the royal son of Hystaspes.
On learning that Darius was with a large army on the left of the Tigris, Alexander hurried forward and crossed that river without opposition. He was at first unable to procure any certain intelligence of the precise position of the enemy, and after giving his army a short interval of rest he marched for four days down the left bank of the river.
A moralist may pause upon the fact that Alexander must in this march have passed within a few miles of the ruins of Nineveh, the great city of the primæval conquerors of the human race. Neither the Macedonian King nor any of his followers knew what those vast mounds had once been. They had already sunk into utter destruction; and it is only within the last few years that the intellectual energy of one of our own countrymen has rescued Nineveh from its long centuries of oblivion.
On the fourth day of Alexander's southward march, his advance guard reported that a body of the enemy's cavalry was in sight. He instantly formed his army in order for battle, and directing them to advance steadily he rode forward at the head of some squadrons of cavalry and charged the Persian horse, whom he found before him. This was a mere reconnoitring party, and they broke and fled immediately; but the Macedonians made some prisoners, and from them Alexander found that Darius was posted only a few miles off, and learned the strength of the army that he had with him. On receiving this news Alexander halted, and gave his men repose for four days, so that they should go into action fresh and vigorous. He also fortified his camp and deposited in it all his military stores and all his sick and disabled soldiers, intending to advance upon the enemy with the serviceable part of his army perfectly unencumbered.
After this halt, he moved forward, while it was yet dark, with the intention of reaching the enemy, and attacking them at break of day. About half way between the camps there were some undulations of the ground, which concealed the two armies from each other's view; but, on Alexander arriving at their summit, he saw, by the early light, the Persian host arrayed before him, and he probably also observed traces of some engineering operation having been carried on along part of the ground in front of them.
Not knowing that these marks had been caused by the Persians having levelled the ground for the free use of their war chariots, Alexander suspected that hidden pitfalls had been prepared with a view of disordering the approach of his cavalry. He summoned a council of war forthwith. Some of the officers were for attacking instantly, at all hazards; but the more prudent opinion of Parmenio prevailed, and it was determined not to advance farther till the battle-ground had been carefully surveyed.
Alexander halted his army on the heights, and, taking with him some light-armed infantry and some cavalry, he passed part of the day in reconnoitring the enemy and observing the nature of the ground which he had to fight on. Darius wisely refrained from moving from his position to attack the Macedonians on the eminences which they occupied, and the two armies remained until night without molesting each other.