On the extreme right were the Medes, once the ruling people of Asia and still mindful of their old renown, the Parthian cavalry, and the sturdy mountaineers of the Caucasus; on the opposite wing were the Bactrians—mostly hardy dwellers in the hills, and famous both for activity and for fierceness—and the native Persians, horse and foot, in alternate formation. But it was in the centre of the line, round the person of Darius, where he stood conspicuous on his royal chariot, that the choicest troops of the Empire were congregated. Here were ranged the Persian Horseguards—a force levied from the noblest families of the race that had ruled Western Asia for more than two centuries. They were known by the proud title of "Kinsmen of the King," and the Footguards, also a cords defile, who carried gold apples at the butt-end of their pikes. Next to these stood the Carians, probably a colony from the well-known people of that name in Asia Minor, possibly transported by some Persian king to a settlement in the East. Of all Asiatic races the Carians had shown themselves the most apt to learn the Greek discipline and to rival Greek valour. Next to the Carians, again, stood the Greek mercenaries.

In front of the line were the scythed chariots, numbering two hundred in all, each with its sharp-pointed sides projecting far beyond the horses, and its sword-blades and scythes stretching from the yoke and from the naves of the wheels. (This is the first time that we hear of the scythed chariot. It was a device of a barbarian kind, and seldom, as far as we know, very effective.) Behind the line, again, was a large mixed multitude, drawn from every tribe that still owned the Great King's sway.

Alexander saw that this time he had a formidable enemy to deal with. He had an entrenched camp constructed, as possibly useful in case of a reverse, and he consulted his generals—a course which he seldom followed —as to how an attack might be most advantageously delivered. But when one of his most experienced officers suggested an assault by night, he emphatically rejected the idea. It was, he declared, an unworthy stratagem; victory so won would be worse than defeat. A more powerful reason was probably the danger of such an attempt. A night attack is always a desperate device.

The first day after coming in sight of the enemy Alexander spent in preparation and consultation. On the morrow he drew out his order of battle. As usual he put himself at the head of the right wing. This was made up of the "Companions," the light infantry, and three out of the six divisions of the phalanx. The left wing, if it may be so called, for there was no centre, consisted of the rest of the phalanx, with a body of cavalry from the allied Greek states.

And now, for the first time, Alexander had a second line in reserve. His numbers were considerably increased, the 35,000 with which he had crossed into Asia having now mounted up to nearly 50,000. And the nature of the battlefield made such an arrangement necessary. The enemy had an enormously superior force and it was necessary to guard against attacks on the flank and the rear. The second line consisted of the light cavalry, the Macedonian archers, contingents from some of the half-barbarous tribes which bordered on Macedonia, some veteran Greek mercenaries and other miscellaneous troops. Some Thracian infantry were detached to guard the camp and the baggage.

The Persians, with their vastly larger numbers, were, of course, extended far beyond the Macedonian line. Left to make the attack, they might easily have turned the flank, or even assailed the rear of their opponents. Alexander, seeing this, and following the tactics which had twice proved so successful, took the offensive. He put himself at the head of the "Companions," who were stationed, as has been said, on the extreme right, and led them forward in person, still keeping more and more to the right, and thus threatening the enemy with the very movement which he had himself reason to dread. He thus not only avoided the iron spikes, which, as a deserter had warned him, had been set to injure the Macedonian cavalry, but almost got beyond the ground which the Persians had caused to be levelled for the operations of their chariots. Fearful at once of being outflanked and of having his chariots made useless, Darius launched some Bactrian and Scythian cavalry against the advancing enemy; Alexander, on the other hand, detached some cavalry of his own to charge the Bactrians, and the action began.

The Bactrians commenced with a success, driving back the Greek horsemen. These fell back on their supports, and advancing again in increased force, threw the Bactrians into confusion. Squadron after squadron joined the fray, till a considerable part of the Macedonian right and of the Persian left wing was engaged. The Persians were beginning to give way, when Darius saw, as he thought, the time for bringing the scythed chariots into action, and gave the word for them to charge, and for his main line to advance behind them. The charge was made, but failed, almost entirely, of its effect. The Macedonian archers and javelin—throwers wounded many of the horses; some agile skirmishers even seized the reins and dragged down the drivers from their places. Other chariots got as far as the Macedonian line, but recoiled from the bristling line of outstretched pikes; and the few whose drivers were lucky enough or bold enough to break their way through all hindrances were allowed to pass between the Macedonian lines, without being able to inflict any serious damage. Then Alexander delivered his counter attack. He ceased his movement to the right. Wheeling half round, the "Companions "dashed into the open space which the advance of the Bactrian squadrons had left in the Persian line. At the same time his own main line raised the and moved forward. Once within the enemy's ranks he pushed straight for the place where, as he knew, the battle would be decided, the chariot of the king. The first defence of that all-important position was the Persian cavalry. Better at skirmishing than at hand-to-hand fighting, it broke before his onslaught. Still there remained troops to be reckoned with who might have made the fortune of the day doubtful, the flower of the Persian foot and the veteran Greeks. For a time these men held their ground; they might have held it longer, perhaps with success, but for the same cause which had brought about the disastrous result of Issus, the cowardice of Darius. He had been dismayed to see his chariots fail and his cavalry broken by the charge of the "Companions," and he lost heart altogether when the dreaded phalanx itself, with its bristling array of pikes, seemed to be forcing a way through the line of his infantry and coming nearer to himself. He turned his chariot and fled, the first, when he should have been the last, to leave his post.

The flight of the king was the signal for a general rout, so far at least as the centre and left wing of the Persian army was concerned. It was no longer a battle; it was a massacre. Alexander pressed furiously on, eager to capture the fugitive Darius. But the very completeness of his victory, it may be said, hindered him. So headlong was the flight that the dust, which, after the months of burning summer heat, lay thick upon the plain, rose like the smoke of a vast conflagration. The darkness was as the darkness of night. Nothing could be seen, but all around were heard the cries of fury and despair, the jingling of the chariot wheels, and the sound of the whips which the terrified charioteers were plying with all their might.

Nor was Alexander permitted to continue the pursuit. Though the Persian left, demoralised by the cowardice of the king, had fled, the right wing had fought with better fortune. It was under the command of Mazæus, who was probably the ablest of the Persian generals, and knew how to use his superiority of numbers. Whilst the sturdy Median infantry engaged the Macedonian front line, Mazæus put himself at the head of the Parthian horse and charged the flank. Parmenio, Alexander's ablest lieutenant—his one general, as he was reported to have said—who was in command, sent an urgent request for help, so hard pressed did he find himself to be. Alexander was greatly vexed, for he saw that all chance of capturing Darius was lost, but he knew his business too well to neglect the demand. He at once called back his troops from the pursuit, and led them to the help of the left wing. Parmenio had sent the same message to that division of the phalanx which had taken part in the advance of the right wing.

As things turned out, however, the help was hardly needed. On the one hand, the Thessalian cavalry had proved themselves worthy of their old reputation as the best horsemen in Greece. Held during the earlier part of the engagement in reserve, they had made a brilliant charge on the Parthians, and had restored the fortune of the day. And then, on the other hand, Mazæus and his men had felt the same infection of fear which the flight of Darius had communicated to the rest of the army. Parmenio felt the vigour of the enemy's attack languish, though he did not know the cause, and had the satisfaction of regaining, and more than regaining, the ground which he had lost, before the reinforcements arrived.

The day was virtually over, yet the hardest fighting of the battle was yet to take place. The Parthian cavalry, with some squadrons of Persian and Indian horse among them, as they retreated across the field of battle, Alexander himself and the "Companions." Their only hope of escape was to cut through the advancing force. It was no time for tactics, only for a desperate charge for life. Each man was fighting for himself, and he fought with a fury that made him a match even for Macedonian discipline and valour. And the enemy had among them some of the most expert swordsmen in the world. Anyhow, the "Companions" suffered more severely than they did in any other engagement in the war. Sixty were slain in the course of a few minutes, three of the principal officers were wounded, and even Alexander himself was in serious danger. But the Parthians thought only of saving their lives, and when they once saw the way clear before them they were only too glad to follow it.

The Persians achieved one more success. A brigade of Indian and Persian horse had plunged through a gap which the movement of the phalanx had left in the line, and attacked the camp. The Thracians who guarded it were hampered by the number of the prisoners whom they had to watch. Many of these escaped. The mother of Darius—the effort had been made for her—might have been one of them, but she refused to go. By this time some troops had come to the rescue of the camp, and the Persian cavalry had to fly.

The great battle of Arbela was over. It was the most hardly won as it was the most conclusive of all Alexander's victories. The Persians made no further stand. The great enemy of Greece had disappeared from the stage of history. But we shall find the powerful forces which Persia represented appear again in another shape.