Muslim conquest of the Levant
Byzantine counterattack: Siege of EmesaEmesa, Syria
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 which particularly came from two cities along the Euphrates river, Circesium and Hīt. The tribes mustered a large army and marched against Emesa in no time, which was erected as military headquarter by Abu Ubaydah at the time.
When the Christian Arabs received the news of the arrival of fresh reinforcements led by the caliph himself, combined with Iyadh invasions of their homeland in Jazira, they immediately abandoned the siege and hastily withdrew there. By the time the Christian Arab coalitions leave, Khalid and his mobile guard has been reinforced by 4000 soldiers under Qa'qa from Iraq, and now has been given permission by Abu Ubaydah to came out of the fort to pursue the enemy. Khalid inflicted heavy losses to the Arab Christian coalition forces, which not only broke the entire siege, but also prevented them to return to Jazira.
The success of the defense, which not only repelled the siege attempt by the Byzantine allies but also allowed Iyadh to capture almost entire Jazira region, has motivated the caliphate to launch the full-scale invasion further to the north until it reached Armenia.