Cyrus rallied the Persian tribes to rise up and overthrow the Median king Astyages by framing the revolt as a fight for freedom from Median domination.

52:26"you are the arbiters of your own fortune. My  voice is the voice of freedom. I am the instrument of your prosperity. Decline all future obedience  to Astyages.” The Persians, who had long spurned at the yoke imposed on them, were glad of such a  leader, and ardently obeyed the call of liberty. All the tribes of the Persian peoples now united  around Cyrus. Astyages was naturally enraged. He ordered that all his court soothsayers,  who had long ago convinced him to spare

53:10and release Cyrus, should be crucified. Then,  with their screams still echoing in his ears, he gathered his armies and marched out to meet  the Persian upstart. As he marched, he sent out messengers, demanding in furious terms that Cyrus  come and bow at his feet, as Herodotus recounts. Cyrus sent the messenger back with the threat  that he would be there a good deal sooner than Astyages liked. Astyages thereupon armed the Medes to a man, and so far lost his wits as to appoint

53:52Harpagus to command them, having apparently  forgotten how he had treated him. The result was that when he took the field and engaged the  Persian army, some of Harpagus' men deserted to the Persians, and the greater number deliberately  shirked fighting and took to their heels. With the betrayal of this general, Astyages  was forced to gather a militia made up of what citizens he could find, and made  a last ditch attempt to stop the army