again. Yazid's corps went to Damascus and captured Beirut. Amr and Shurahbil's corps went on to conquer the rest of Palestine, while Abu Ubayda and Khalid, at the head of a 17,000 strong army moved north to conquer whole of the northern Syria. Abu Ubayda sent the commanders 'Amr ibn al-'As, Yazid ibn Abi Sufyan, and Shurahbil ibn Hassana back to their areas to reconquer them. Most of the areas submitted without a fight. Abu Ubayda himself, along with Khalid, moved to northern Syria once again to conquer them with a 17,000 strong army. Khalid along with his cavalry was sent to Hazir and Abu Ubayda moved to Chalcis. Conquest of northern Syria With Emesa already in hand, Abu Ubayda and Khalid moved towards Chalcis, which was strategically the most significant Byzantine fort. Defense of Emesa After the devastating defeat in the Battle of 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 Heraclius thus sought help from the Christian Arab tribes which came of Jazirah that mainly from Circesium and Hīt and the tribes mustered a large army and marched to besiege Emesa. Abu Ubayda withdrew all his forces from northern Syria to Emesa as a part of complex strategy which he devised to repel the massive invasion of the Christian Arabs against Emesa, while Caliph Umar instructed Abu Ubaydah to send his field commanders outside of Emesa with sufficient splinter forces to lay counter siege to cities in Jazira, homeland of enemy Arab christian tribes, in order to divert the focus of enemy concentration in Emesa. So the splinter forces under Iyadh ibn Ghanm In 638 the Muslims attacked Hīt, which they found to be well fortified; thus, they left a fraction of the army to impose a siege on the city, while the rest went after Circesium. After Emesa Later on, In Chalcis, the Byzantines would guard Anatolia, Heraclius's homeland Armenia and there the Asian zone's capital Antioch. Abu Ubayda sent Khalid, with his elite cavalry, the mobile guard, towards Chalcis. The fort was guarded by the Greek troops under their commander, Menas, who was reported to be of high prestige, second only to the emperor himself.