When Emessa was still under siege, Heraclius the Byzantine emperor made another bid to muster strength and drive away the Muslims from the land of Syria. This time he planned action on a massive scale. By May 636 A.D., a Byzantine army of 150,000 men had been put in arms and concentrated at Antioch.
At this time the Muslims were operating in four pockets. Amr b. Al Aas was operating with his corps in Palestine; Shurahbil was in Jordan; Yazeed was in Caesara, while Abu Ubaida and Khalid were at Emessa.
The plan of the Byzantines was that one Byzantine force was to march from Damascus from the west, and cut off the Muslim force at Emessa. Another force was to attack the Muslims at Emessa from the north. One force was to attack Emessa from the east and still another from the west. The plan was to recapture Emessa and Damascus.
When the Muslims came to know of the Byzantine plan they held a council of war. The Muslims decided that instead of being divided into four pockets, they snould consolidate their forces at one point and face the Byzantines as a united force.
The next point for consideration was where should the Muslim forces concentrate? If the Muslims concentrated their forces in North Syria, they were apt to be surrounded by the Byzantine forces and their contact with the Arabian desert was likely to be cut off. The only strategy under the circumstances was that the Muslims should concentrate their forces in southern Syria where they could always maintain contact with Arabia. In accordance with this decision, the Muslims vacated Emessa, Damascus and other posts in North Syria, and concentrated their forces at Jabiya in Yermuk valley to the south of Damascus.
When the Byzantine force reached Emessa they found that the Muslims had left. They found that Damascus had also been evacuated. The Byzantines marched to the south and reached the Yermuk valley some time in the third week of July 636. Here they settled down in camps, and began their preparations for a confrontation with the Muslims. The Byzantine camp was 18 miles long, and between the Byzantine camp and the Muslim camp lay the central parts of the plain of Yermuk. The Byzantine forces comprised of 2 lakh men fully equipped.
The Muslim army consisted of 40,000 men. Against every five Byzantine soldiers there was only one Muslim soldier. When the Byzantine Generals surveyed their army, they felt sure of their victory.
The Muslims were fired with their faith, and hoped that God would grant them victory in spite of the odds against them. Abu Ubaida felt that it was going to be a tough battle. He thought that at that critical stage it was necessary to avail of the military skill of Khalid. Abu Ubaida accordingly decided to remain the nominal Commander-in-Chief. He delegated his powers of field operations to Khalid.
For some time there were negotiations between the two parties. The Byzantines offered to pay the Muslims some money in case they left Syria and returned to Arabia. The Muslims spurned the offer. In return the Muslims offered the Byzantines the usual three alternatives, Islam, Jizya or the sword.