[5] 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.
[6] Iyadh in turn sent Habib ibn Maslama al-Fihri toward Circesium taking with him a particular division of Banu Kilab known as Corps of 'Amr,[7] which are famed for their 'militant and warlike attitude.
Between the end of 639 and December 640, Iyad and his lieutenants subdued, in succession, Circesium (al-Qarqisiya), Amid, Mayyafariqin, Nisibin, Tur Abdin, Mardin, Dara, Qarda and Bazabda.
[11] When the Christian Arabs received the news of the arrival of fresh reinforcements led by the caliph himself,[1] combined with Iyadh invasions of their homeland in Jazira,[8][1] they immediately abandoned the siege and hastily withdrew there.
[1][12] By the time the Christian Arab coalitions left, Khalid and his mobile guard has been reinforced by 4000 soldiers under Qa'qa from Iraq, and now had been given permission by Abu Ubaydah to came out of the fort to pursue the enemy.
[14][15] 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.