After the battle of Marj-ur-Rum, the Muslim forces under Khalid advanced to Emessa in the north and laid siege to the city. After some time, Abu Ubaida also arrived at Emessa along with the rest of the Muslim army. The citizens of Emessa thereupon felt that they were no match for the Muslim forces. They asked for a truce which was allowed. The people of Emessa paid 10,000 diners and 100 robes of brocade. The truce was stipulated for a period of one year during which period the Muslims were not to attack them. If Emessa received any reinforcerment during this period, the citizens of Emessa could resume hostilities. After the truce was agreed upon the gates of the city of Emessa were thrown open, and the Muslims were free to move in the city.

After the truce with Emessa, the Muslims attacked the neighbouring cities, and these cities also sought truce on the lines of the truce of Emessa. In the meantime the winter set in. Heraclius sent considerable force to reinforce the garrison of Emessa. With the arrival of reinforcement the truce ended and the hostilities were resumed.

The military Governor of Emessa was Harbees, and to him Heraclius wrote, "The Muslims cannot stand the cold of Syria. Fight them on every cold day so that none of them is left till the spring."

For some time the siege of Emessa continued with unbroken monotony. Every day there was an exchange of archery, but there was no major action. The Byzantines hoped that the severe cold of Syria would be enough to destroy the desert dwellers and drive them away. The Muslims, however, withstood the cold with great resoluteness. By March 636 the severity of the cold was over, and the hopes of the Byzantines that the cold would drive away the Muslims were dashed to the ground. The Byzantines now became desperate. One morning a gate of the city was flung open, and Harbees the Byzantime Commander led a surprise attack against the unsuspecting Muslims. In the momentum of the surprise attack the Byzantines moved forward and the Muslims were forced to fall back.

At this juncture, Abu Ubaida commissioned Khalid to go to the relief of the Muslims. Khalid regrouped the Muslim army, and launched a counter attack. By sunset the Byzantines were forced back inside the city. The Byzantines had fought hard, and the Muslims felt that Harbees was no ordinary Commander; he was a force to be reckoned with.

The following morning, Abu Ubaida held a council of war. Most of the Muslim soldiers were in a restrained mood. Khalid advised that they should stage a withdrawal. When the sun rose, the Muslims had packed their belongings, struck the tents and had begun the withdrawal. Harbees thought that his action during the previous day had unnerved the Muslims, and they had accordingly raised the siege.

Harbees felt elated and he thought of giving a beating to the retreating Muslims. Harbees launched his mounted force into a fast pursuit to catch up with the retreating Muslims. The Muslims increased their pace, and the Byzantines also quickened the pursuit. When they were sufficiently away from Emessa, Khalid gave the signal, and the Muslim forces rushed from all sides to surround the Byzantines. Steadily closing in from all sides, the Muslims struck the Byzantines with spears and swords. The Byzantines fought desperately but were slaughtered down in large numbers. Breaking through the Byzantine force, Khalid reached Harbees, and then a duel began between the two Generals. In this duel the sword of Khalid broke and for some time Khalid was at the mercy of Harbees. Khalid held Harbees tight in his grip and then with his steel like grip splintered his ribs. Harbees fell lifeless in the hands of Khalid. The death of Harbees was the signal for the end of the Byzantine resistance.

The Muslims marched back to Emessa triumphant. There was no further resistance at Emessa. The citizens surrendered on the usual terms and the city of Emessa was occupied by the Muslims towards the closing days of March 636.