After the Ridda Wars a Muslim tribal chief raided the Persian frontier towns in Iraq. After the success of these raids, Abu Bakr planned to expand his empire. He started with Iraq, which was under Sassanid occupation at the time. It was important for Abu Bakr that his expedition did not suffer a defeat, for that would confirm and strengthen the fear of Sassanid military strength. To overcome these concerns, he decided that the army that would battle the Persians would consist entirely of volunteers. He put in command of the army his best general, Khalid ibn al-Walid.