When the Muslims were busy operating in the Jordan sector, Heraclius thought that it was a good opportunity to attack Damascus and recapture it. Heraclius accordingly sent from Antioch a strong Byzantine force under a General named Theodorus to recapture Damascus.

When Abu Ubaida came to know that a Byzantine force was marching to Damascus, the Muslim contingents led by Abu Ubaida and Khalid left Fahl and moved northward to Damascus. When Heraclius came to know of the movements of the Muslim force, he ordered a detachment of the Byzantine forces stationed at Emessa north of Damascus to be sent to Damascus to reinforce the Byzantine forces under Theodorus.

The Muslim and Byzantine forces met in the plain of Marj-ur-Rum to the west of Damascus. As the two armies stood in battle formation, the Byzantine contingent from Emessa under the command of Shans faced the corps of Abu Ubaida, while the corps of Khalid faced the main army commanded by Theodorus. The two armies remained in their battle positions, each waiting for the other to make the first move. No side made the move.

As night fell, leaving Shans to face the Muslims, Theodorus pulled back his corps under the cover of darkness, and by dawn the following day arrived at Damascus. The movement was carried out with great skill and it was only about dawn that the Muslims came to know that the bulk of the Byzantine army had left for Damascus.

The Muslim garrison at Damascus was under the command of Yazeed. On coming to know that a Byzantine force had arrived under the command of Theodorus, Yazeed immediately deployed his force outside the fort. Just after sunrise the battle began between the Muslims and the Byzantines. The Byzantines vastly outnumbered the Muslims, and in spite of the strong pressure of the Byzantines the Muslims held fast till noon. Thereafter the Byzantines increased their pressure and the force under Yazeed began to fall back.

At that juncture Byzantines were struck in their rear by a furious mass of Muslim horsemen. This was the mobile guard led by Khalid. When Khalid came to know that the force under Theodorus facing him was no longer at Marj-ur-Rum he guessed that the force must have proceeded to Damascus. Khalid accordingly rushed with his mobile guard to the relief of Damascus. So great was the onslaught of the Muslim cavalry that the Byzantine forces wedged in between the forces of Yazeed and Khalid were cut to pieces.

Khalid killed Theodorus. With his death the Byzantine forces lost nerve. They retired from the battle-field in great confusion. Thousands of them were killed. Only a few escaped to tell the tale of disaster. The battle of Damascus ended in a victory for the Muslims.

At Marj-ur-Rum the Muslims under Abu Ubaida faced the Byzantines under Shans. The battle began with a personal duel between Abu Ubaida and Shans. In this duel, Abu Ubaida killed Shans. Thereafter the battle began, and both the sides appeared to be balanced. By afternoon news was received that in Damascus the Byzantine force had been shattered. That unnerved the Byzantine force fighting at Marj-ur-Rum. As night fell, the Byzantines fell back and retreated to Emessa. The battle of Marj-ur-Rum was fought in March 635 A.D.