His great antagonist came on across the Euphrates against him, at the head of an army which Arrian, copying from the journals of Macedonian officers, states to have consisted of forty thousand foot, and seven thousand horse. In studying the campaigns of Alexander, we possess the peculiar advantage of deriving our information from two of Alexander's generals of division, who bore an important part in all his enterprises. Aristobulus and Ptolemy (who afterwards became king of Egypt) kept regular journals of the military events which they witnessed; and these journals were in the possession of Arrian, when he drew up his history of Alexander's expedition. The high character of Arrian for integrity makes us confident that he used them fairly, and his comments on the occasional discrepancies between the two Macedonian narratives prove that he used them sensibly. He frequently quotes the very words of his authorities: and his history thus acquires a charm such as very few ancient or modern military narratives possess. The anecdotes and expressions which he records we fairly believe to be genuine, and not to be the coinage of a rhetorician, like those in Curtius. In fact, in reading Arrian, we read General Aristobulus and General Ptolemy on the campaigns of the Macedonians; and it is like reading General Jomini or General Foy on the campaigns of the French.
The estimate which we find in Arrian of the strength of Alexander's army, seems reasonable when we take into account both the losses which he had sustained, and the reinforcements which he had received since he left Europe. Indeed, to Englishmen, who know with what mere handfuls of men our own generals have, at Plassy, at Assaye, at Meeanee, and other Indian battles, routed large hosts of Asiatics, the disparity of numbers that we read of in the victories won by the Macedonians over the Persians presents nothing incredible. The army which Alexander now led was wholly composed of veteran troops in the highest possible state of equipment and discipline, enthusiastically devoted to their leader, and full of confidence in his military genius and his victorious destiny.
The celebrated Macedonian phalanx formed the main strength of his infantry. This force had been raised and organized by his father Philip, who on his accession to the Macedonian throne needed a numerous and quickly-formed army, and who, by lengthening the spear of the ordinary Greek phalanx, and increasing the depth of the files, brought the tactic of armed masses to the greatest efficiency of which it was capable with such materials as he possessed. [See Niebuhr's Hist. of Rome, iii. 488.] He formed his men sixteen deep, and placed in their grasp the SARISSA, as the Macedonian pike was called, which was four-and-twenty feet in length, and when couched for action, reached eighteen feet in front of the soldier: so that, as a space of about two feet was allowed between the ranks, the spears of the five files behind him projected in advance of each front-rank man. The phalangite soldier was fully equipped in the defensive armour of the regular Greek infantry. And thus the phalanx presented a ponderous and bristling mass, which as long as its order was kept compact, was sure to bear down all opposition. The defects of such an organization are obvious, and were proved in after years, when the Macedonians were opposed to the Roman legions. But it is clear that, under Alexander, the phalanx was not the cumbrous unwieldy body which it was at Cynoscephalae and Pydna. His men were veterans; and he could obtain from them an accuracy of movement and steadiness of evolution, such as probably the recruits of his father would only have floundered in attempting, and such as certainly were impracticable in the phalanx when handled by his successors: especially as under them it ceased to be a standing force, and became only a militia. [See Niebuhr.] Under Alexander the phalanx consisted of an aggregate of eighteen thousand men, who were divided into six brigades of three thousand each. These were again subdivided into regiments and companies; and the men were carefully trained to wheel, to face about, to take more ground, or to close up, as the emergencies of the battle required. Alexander also arrayed in the intervals of the regiments of his phalangites, troops armed in a different manner, which could prevent their line from being pierced, and their companies taken in flank, when the nature of the ground prevented a close formation; and which could be withdrawn, when a favourable opportunity arrived for closing up the phalanx or any of its brigades for a charge, or when it was necessary to prepare to receive cavalry.
Besides the phalanx, Alexander had a considerable force of infantry who were called shield-bearers: they were not so heavily armed as the phalangites, or as was the case with the Greek regular infantry in general; but they were equipped for close fight, as well as for skirmishing, and were far superior to the ordinary irregular troops of Greek warfare. They were about six thousand strong. Besides these, he had several bodies of Greek regular infantry; and he had archers, slingers, and javelin-men, who fought also with broadsword and target. These were principally supplied to him by the highlanders of Illyria and Thracia. The main strength of his cavalry consisted in two chosen corps 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 armour. 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 Codomannus 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 remains of Nineveh, the great, city of the primaeval 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 become nameless masses of grass-grown ruins; 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. [See Layard's "Nineveh," and also Vaux's "Nineveh and Persepolis," p. 16.]
On the fourth day of Alexander's southward march, his advanced 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 eminences which they occupied, and the two armies remained until night without molesting each other. On Alexander's return to his head-quarters, he summoned his generals and superior officers together, and telling them that he well knew that THEIR zeal wanted no exhortation, he besought them to do their utmost in encouraging and instructing those whom each commanded, to do their best in the next day's battle. They were to remind them that they were now not going to fight for a province, as they had hitherto fought, but they were about to decide by their swords the dominion of all Asia. Each officer ought to impress this upon his subalterns and they should urge it on their men. Their natural courage required no long words to excite its ardour: but they should be reminded of the paramount importance of steadiness in action. The silence in the ranks must be unbroken as long as silence was proper; but when the time came for the charge, the shout and the cheer must be full of terror for the foe. The officers were to be alert in receiving and communicating orders; and every one was to act as if he felt that the whole result of the battle depended on his own single good conduct.
Having thus briefly instructed his generals, Alexander ordered that the army should sup, and take their rest for the night.