The Secret Weapon of Central Asian Nomads

Darius had to quell a revolt against central asian people. The secret weapon of these people darius tries to conquer is that there's nothing to conquer. They're nomads. There are no cities. And how you, with your fantastic army, deal with them, if there's strategy to deal with you, i's to simply retreat in front of you and keep going while your supply lines get longer and longer and longer. Ask napoleon how well that can work.