Well, most of Ancient battles with huge armies was decisive. The losser would just fall apart. Either its army would just fell to the last man or the remaining forces would scarce around after the troops lost confidence to their leader. Darius had 3 major battles against Alexander. In Granicus he fielded about 30.000 men, in Issus 55.000 and in Gaugamela about 500.000.After every succesive defeat he had to gather new troops for the next battle. When he lost in Gaugamela about 30 to 40 thousand Persians (and asian allies) were killed in the field. But the bulk of his army scattered around after seeing him fleeing. Only his bodyguards remained by his side.