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80 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 troops armed in a different manner in the intervals of the regiments of his phalangites, who could prevent their line from being pierced, and their companies taken in flank, when the nature of the ground prevented a close formation, and who could be withdrawn when a favorable opportunity arrived for closing up the phalanx or any of its brigades for a charge, or when it was necessary to prepare to receive cavalry.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, and who were principally supplied him by the highlanders of Illyria and Thracia. 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 Mazus 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.