<P> I would not sheathe what Allah had unsheathed against the disbelievers . </P> <P> Khalid then crushed the most powerful threat to the nascent Islamic state of Medina: Musaylimah, a claimant to prophethood, who had already defeated two Muslim armies . In the third week of December 632, Khalid won a decisive victory against Musaylimah at the Battle of Yamama . Musaylimah died in the battle, and nearly all resistance from rebelling tribes collapsed . </P> <P> With the collapse of the rebellion, and Arabia united under the central authority of the caliph at Medina, Abu Bakr decided to expand his empire . It is unclear what his intentions were, whether it was a full scale expansion plan or pre-emptive attacks to secure more territory to create a buffer zone between the Islamic state and the powerful Sassanid and Byzantine empires . Khalid was sent to the Persian Empire with an army consisting of 18,000 volunteers to conquer the richest province of the Persian empire, Euphrates region of lower Mesopotamia, (present day Iraq). Khalid entered lower Mesopotamia with this force . It is recorded that before engaging the Sassanid forces Khalid wrote a letter to taunt his opponents to surrender: </P> <P> Submit to Islam and be safe . Or agree to the payment of the Jizya (tax), and you and your people will be under our protection, else you will have only yourself to blame for the consequences, for I bring the men who desire death as ardently as you desire life . </P>

Who was once a commander of the muslim army and later become an ordinary soldier