Alexander's audacity was largely due to the support of his army, which had been formed during the reign of Philip II. The strength of this army was the phalanx and its cavalry. The phalanx consisted of 3,000 foot guards and six divisions of 1,500 men each. The soldiers did not wear upper body armor; they were very vulnerable but had great mobility. They used small shields that did not have handles, so they held a 5-meter-long spear with both hands, the sarissa.

They also had short swords for hand-to-hand combat. The cavalry, which always formed the right wing of Alexander's army, consisted of eight battalions of cavalry, each battalion of about 1,800 men or more, drawn from the Macedonian noble families. They wore armor and helmets and carried spears, swords, and sarissas.