Alexander's operations with his right and centre had exposed his left to an immensely preponderating force of the enemy. Parmenio kept out of action as long as possible; but Mazaeus, who commanded the Persian right wing, advanced against him, completely outflanked him, and pressed him severely with reiterated charges by superior numbers. Seeing the distress of Parmenio's wing, Simmias, who commanded the sixth brigade of the phalanx, which was next to the left wing, did not advance with the other brigades in the great charge upon the Persian centre, but kept back to cover Parmenio's troops on their right flank; as otherwise they would have been completely surrounded and cut off from the rest of the Macedonian army. By so doing, Simmias had unavoidably opened a gap in the Macedonian left centre; and a large column of Indian and Persian horse, from the Persian right centre, had galloped forward through this interval, and right through the troops of the Macedonian second line. Instead of then wheeling round upon Sarmenio, or upon the rear of Alexander's conquering wing, the Indian and Persian cavalry rode straight on to the Macedonian camp, overpowered the Thracians who were left in charge of it, and began to plunder. This was stopped by the phalangite troops of the second line, who, after the enemy's horsemen had rushed by them, faced about, countermarched upon the camp, killed many of the Indians and Persians in the act of plundering, and forced the rest to ride off again. Just at this crisis, Alexander had been recalled from his pursuit of Darius, by tidings of the distress of Parmenio, and of his inability to bear up any longer against the hot attacks of Mazaeus. Taking his horse-guards with him, Alexander rode towards the part of the field where his left wing was fighting; but on his way thither he encountered the Persian and Indian cavalry, on their return from his camp.