With the devastating defeat at Yarmouk his empire was extremely vulnerable to Muslim invasion. With few military resources left he was no longer in a position to attempt a military come back in Syria. To gain time for the preparations of the defense of the rest of his empire, Heraclius needed the Muslims occupied in Syria. He sought help of the Christian Arabs of Al-Jazira who mustered up a large army and marched against Emesa, Abu Ubaidah's headquarters. Abu Ubaidah withdrew all his forces from northern Syria to Emesa, and the Christian Arabs laid siege to the city. Khalid was in favour of an open battle outside the fort, but Abu Ubaidah rather sent the matter to Umar, who handled it brilliantly. Umar sent detachments of the Muslim army from Iraq to invade Al-Jazira, homeland of the invading Christian Arabs, from three different routes. Moreover, another detachment was sent to Emesa from Iraq under Qa’qa ibn Amr, a veteran of Yarmouk who was sent to Iraq for the Battle of al-Qādisiyyah. Umar himself marched from Medina at the head of 1,000 men. The Christian Arabs, when they received the news of the Muslim invasion of their homeland, abandoned the siege and hastily withdrew to Al-Jazira. At this point Khalid and his mobile guard came out of the fort and devastated the army, attacking them from the rear.