While Khalid was clearing the region of Eastern Syria, Abu Ubaidah came to know that he would come under Khalid's command upon the latter's arrival. He decided to take Bosra quickly. He therefore sent Shurahbil with 4,000 men to capture Bosra. Shurahbil marched to Bosra, the garrison of which withdrew into the fortified town as soon as the Muslims appeared in sight. This garrison consisted of 4,000 soldiers, but expecting that more Muslim forces would soon arrive and that Shurahbil's detachment was only an advance guard, it remained within the walls of the fort. Shurahbil camped on the western side of the town, and positioned groups of his men all round the fort.