...Actually, the fate of Darius is due to the classic foolishness in having a disarmed populace. Although the chance of rebellion was lessened due to the emasculation of the subject peoples of the Persian empire, their ability to respond to the call for mass levies with trained military manpower was eliminated as well. The person talking of farmers in uniform comprising almost all the Persian army was half-right: they didn't wear uniforms.
...Even a Hannibal in charge of the untrained rabble at Gaugamela would have had a hard time winning.