the border between Arabia and these two great empires were numerous nomadic Arab tribes serving as a buffer between the Persians and Romans. Abu Bakr hoped that these tribes might accept Islam and help their brethren in spreading it. Second, the Persian and Roman populations were very highly taxed; Abu Bakr believed that they might be persuaded to help the Muslims, who agreed to release them from the excessive tributes. Finally, Abu Bakr hoped that by attacking Iraq and Syria he might remove the danger from the borders of the Islamic State. 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 used a volunteer army and 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 Sasanian 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. After entering Mesopotamia, he dispatched messages to every governor and deputy who ruled the provinces. The messages said; “In the Name of God, the Most Compassionate and Merciful. Khalid ibn Walid sends this message to the satraps of Persia. Peace will be upon him who follows the guidance. All praise and thanks be to God who disperses your power and thwarted your deceitful plots. On the one hand, he who performs our prayers facing the direction of our Qiblah to face the sacred Mosque in Mekkah and eats our slaughtered animals is a Muslim. He has the same rights and duties that we have. On the other hand, if you