Caliph Abu Bakr sent his four corps under Amr ibn al-A'as, Abu Ubaidah ibn al-Jarrah, Shurahbil ibn Hasana, and Yazid ibn Abi Sufyan and appointed for them different districts of Syria to capture. They were unable to get significant success in their goals and were under great pressure because of the concentration of the Byzantine army at Ajnadayn. Abu Bakr therefore decided to send Khalid ibn Walid, the conqueror of Iraq, to Syria to command the Rashidun army there. Khalid ibn Walid reached Syria and capturing town after town he finally reached the city of Bosra in June 634 A.C. According to his instructions, Abu Ubaidah ibn al-Jarrah who had already occupied the District of Hauran which lay north-east of the river Yarmuk, was to remain at his position until Khalid arrived at Bosra. Abu Ubaidah ibn al-Jarrah had three corps of the Muslim army under his command - his own, Yazid's, and Shurahbil's, but he had fought no battles and captured no towns. One place which worried him a great deal was Bosra, a large town which was the capital of the Ghassanid Kingdom. It was garrisoned by a strong force of Byzantine and Christian Arabs under the command of Roman officers.