Behind this main line, was placed a second or body of reserve, intended to guard against attacks in the flanks and rear, which the superior numbers of the Persians rendered probable. For this purpose, Alexander reserved,—on the right, the light cavalry or Lancers—the Pæonians, under Aretes and Aristo—half the Agrianes, under Attalus—the Macedonian archers, under Brisson—and the mercenaries of old service, under Kleander; on the left, various bodies of Thracian and allied cavalry, under their separate officers. All these different regiments were held ready to repel attack either in flank or rear. In front of the main line were some advanced squadrons of cavalry and light troops—Grecian cavalry, under Menidas on the right, and under Andromachus on the left—a brigade of darters under Balakrus, together with Agrianian darters, and some bowmen. Lastly, the Thracian infantry were left to guard the camp and baggage.[381] [381] Arrian, iii. 12, 2-6; Curtius, iv. 13, 30-32; Diodor. xvii. 57. Forewarned by a deserter, Alexander avoided the places where iron spikes had been planted to damage the Macedonian cavalry.[382] He himself, at the head of the Royal Squadron, on the extreme right, led the march obliquely in that direction, keeping his right somewhat in advance. As he neared the enemy, he saw Darius himself with the Persian left centre immediately opposed to him—Persian guards, Indians, Albanians, and Karians. Alexander went on inclining to the right, and Darius stretching his front towards the left to counteract this movement, but still greatly outflanking the Macedonians to the left. Alexander had now got so far to his right, that he was almost beyond the ground levelled by Darius for the operations of his chariots in front. To check any farther movement in this direction, the Baktrian 1000 horse and the Scythians in front of the Persian left, were ordered to make a circuit and attack the Macedonian right flank. Alexander detached against them his regiment of cavalry under Menidas, and the action thus began.[383] [382] Curtius, iv. 13, 36; Polyænus, iv. 3, 17. [383] Arrian, iii. 13, 1-5. The Baktrian horse, perceiving the advance of Menidas, turned from their circuitous movement to attack him, and at first drove him back until he was supported by the other advanced detachments—Pæonians and Grecian cavalry. The Baktrians, defeated in their turn, were supported by the satrap Bessus with the main body of Baktrians and Scythians in the left portion of Darius’s line. The action was here for some time warmly contested, with some loss to the Greeks; who at length however, by a more compact order against enemies whose fighting was broken and desultory, succeeded in pushing them out of their place in the line, and thus making a partial opening in it.[384] [384] Arrian, iii. 13, 9.