Following his defeat at the hands of Alexander at the battle of Issus in 333 BCE, Darius III withdrew to Babylon. Here he worked to rebuild his army and gather his forces from across the empire.

considering what the Achaemenid Empire was capable of. Ancient Hellenistic sources report a (clearly inflated) army of 40,000 infantry and 700 cavalry. Modern estimates place his forces at 700-2,000.