The Persians had long been discontent that the Medes ruled them, and now having got a champion they were glad to win their freedom. But when Astyages heard that Cyrus was about this business, he sent a messenger to summon him;

Cyrus told the messenger to take back word that Astyages would see him sooner than he liked. Hearing this, Astyages armed all his Medes, and was distracted by Providence so that he forgot what he had done to Harpagus, and appointed him to command the army.

So when the Medes marched out and engaged with the Persians, those who were not in on the plan fought, while others deserted to the enemy, and most were deliberate cowards and ran.