Jazira was the land in upper reaches of the Tigris and the Euphrates. It was populated by tribes professing the Christian faith. When Syria was lost to the Christian Byzantines, the Christian tribes of Jazira persuaded Heraclius the Byzantine emperor to make another attempt to drive away the Muslims from Syrian soil. In this task the Christian tribes of Jazira offered to come to the assistance of the Byzantine forces.
Heraclius welcomed the offer. A large Byzantine army was assembled and it was led to Syria to have another confrontation with the Muslims. Large contingents of Christian tribes crossed over to Syria from Jazira. All these armies headed towards Emessa. The strategy of the Christians was to occupy Emessa which was the key to North Syria.
In view of the Christian pressure, Abu Ubaida mustered all Muslim forces in North Syria at Emessa, and decided to play the defensive role. When the Christian forces came to Emessa they found the gates of the city of Emessa shut against them.
Abu Ubaida wrote to Umar and asked for reinforcements. Umar had established military cantonments in important cities where reserves were available for mobilisation in the case of emergency. Umar despatched fleet couriers to selected cantonments requiring the commanders of the stations to despatch reinforcements to Emessa immediately.
Qaqaa b. Amr was stationed at Kufah. Umar directed him to hasten with 4,000 cavalry to Emessa. Umar himself proceeded with some force from Madina. The Caliph himself stayed at Damascus, but he sent the force to Emessa.
As the Christian tribes from Jazira were the main component of the invading army, Umar decided to launch an attack against Jazira. Suhail b. Adi was directed to dash to Jazira and attack the Christian tribes at Emessa from the rear. Abdullah b. Utban was asked to lead a contingent to Nisibin and launch an attack at the heart of Jazira. Walid bin Uqbah was deputed on a diplomatic mission to Jazira to negotiate with the Arab tribes in Jazira, and persuade them to withdraw their support from the Byzantines.
At Emessa, Abu Ubaida held a council of war. The point for consideration was whether the Muslims should sally forth from the city and give the enemy a fight, or whether they should remain locked up in the city and watch further develop" meets. Khalid bin Walid was in favour of an offensive and giving the enemy a fight. The contrary view was that for the time being the Muslims should remain on the defensive, and an attack should be launched when an adequate reinforcement had been received Abu Ubaida decided to await further aid.
When Jazira itself was attacked by Muslim contingents under Suhail b. Adi and Abdullah b. Utban the warriors of the Christian tribes at Emessa decided to return to Jazira to protect their own homes and hearths. Negotiations with the Arab tribes at Jazira also bore fruit and they agreed to support their Arab brethren instead of the Byzantines.
At this stage the Muslim forces in the city of Emessa sallied forth from their fortification and dashed against the Christian forces that were still there. The Christians could not withstand the attack, and they beat retreat after suffering heavy losses. The Christian army was routed and they took to flight. That was the last battle of the Byzantines on the Syrian soil. Syria was now completely under the domination of the Muslims.