After the Ridda wars, a tribal chief of northeastern Arabia, Al-Muthanna ibn Haritha, raided the Sasanian towns in Mesopotamia (modern-day Iraq). With the success of the raids, a considerable amount of booty was collected. Al-Muthanna ibn Haritha went to Medina to inform Abu Bakr about his success and was appointed commander of his people, after which he began to raid deeper into Mesopotamia. Using the mobility of his light cavalry, he could easily raid any town near the desert and disappear again into the desert, beyond the reach of the Sasanian army. Al-Muthanna's acts made Abu Bakr think about the expansion of the Rashidun Empire.

To ensure victory, Abu Bakr made two decisions concerning the attack on Persia: first, the invading army would consist entirely of volunteers; and second, to put his best general, Khalid ibn al-Walid, in command. After defeating the self-proclaimed prophet Musaylimah in the Battle of Yamama, Khalid was still at Al-Yamama when Abu Bakr ordered him to invade the Sassanid Empire. Making Al-Hirah the objective of Khalid, Abu Bakr sent reinforcements and ordered the tribal chiefs of northeastern Arabia, Al-Muthanna ibn Haritha, Mazhur bin Adi, Harmala and Sulma to operate under Khalid's command. Around the third week of March 633 (first week of Muharram 12th Hijrah) Khalid set out from Al-Yamama with an army of 10,000. The tribal chiefs, with 2,000 warriors each, joined him, swelling his ranks to 18,000.