Khalid was expecting that he would have to fight for Hirah. Therefore, instead of approaching Hirah from the front, Khalid made a detour and approached Hirah from the rear.
When the Muslims reached the gates of the city, there was no Persian army to oppose them. When Azarbeh came to know of the death of his son, he was smitten with grief. In the meantime Ardsheer the emperor of Persia died and Persia came to be rocked by succession disputes. That unnerved Azarbeh.
He abandoned Hirah, and fled with the Persian forces to Madain. Under the circumstances, the Muslims occupied the city of Hirah without any resistance.