After the devastating defeat at Yarmouk, the remainder of the Byzantine empire was left vulnerable. With few military resources left, it was no longer in a position to attempt a military comeback in Syria. To gain time to prepare a defense of the rest of his empire, Heraclius needed the Muslims occupied in Syria. He thus sought help from the Christians (some of whom were Arabs) of Jazirah, mainly from Circesium and Hīt, who mustered a large army and marched against Emesa, Abu Ubaidah's headquarters. Abu Ubaidah withdrew all his forces from northern Syria to Emesa, and the Christians laid in a siege. Khalid was in favor of an open battle outside the fort, but Abu Ubaidah referred the matter to Umar, who sent a detachment from Iraq to invade Jazirah from three different routes. Another detachment was sent to Emesa from Iraq under Qa’qa ibn Amr, a veteran of Yarmouk, who had originally been sent to Iraq for the Battle of al-Qādisiyyah. Umar himself marched from Medina with 1,000 men.