Companion cavalry
Initially, Alexander chose what was apparently unfavorable ground. This surprised Darius who mistakenly elected to hold the wrong position while Alexander instructed his infantry to take up a defensive posture. Alexander personally led the more elite Greek Companion cavalry against the Persian left up against the hills, and cut up the enemy on the less encumbering terrain, thereby generating a quick rout. After achieving a breakthrough, Alexander demonstrated he could do the difficult thing and held the cavalry in check after it broke the Persian right. Alexander then mounted his beloved horse Bucephalus, took his place at the head of his Companion cavalry, and led a direct assault against Darius. The horses that were pulling Darius' chariot were injured, and began tossing at the yoke. Darius, about to fall off his chariot, instead jumped off. He threw his royal diadem away, mounted a horse, and fled the scene. The Persian troops, realizing they had lost, either surrendered or fled with their hapless king. The Macedonian cavalry pursued the fleeing Persians for as long as there was light. As with most ancient battles, significant carnage occurred after the battle as pursuing Macedonians slaughtered their crowded, disorganized foe.