After the failure of negotiations, the arbitration was left to the sword. The battle began in the third week of August 636.
Both the armies faced each other across the plain of Yermuk, about a mile apart.
Before the two armies clashed, a Byzantine General George emerged from the Byzantine centre and rode towards the Muslims. Approaching the Muslim centre he asked for Khalid. Khalid rode out thinking that Geerge wanted to have a duel with him. But George had no intention to duel. Instead George asked a few questions about Islam, and the Holy Prophet. He also enquired as to why Khalid was called 'The Sword of Allah'. Khalid answered these questions, and George said that he was satisfied. Khalid thereupon invited George to accept Islam and declare the article of faith Surprisingly enough, George accepted the invitation and was converted to Islam at the hands of Khalid. Then George rode to the Muslim side where he was welcomed with great enthusiasm.
The Commander-in-chief of the Byzantine force felt much annoyed at the walk over of his General George to the Muslim camp. He vowed vengeance against the Muslims as well as George. In a fit of fury he chose a few selected warriors from the Byzantine side, and they challenged the Muslims to duel Scores of duels were fought on the plain of Yermuk. Practically all the Byzantine champions were killed in the combat. On the Muslim side honours went to Abdur Rahman the son of Abu Bakr who killed five Byzantine champions one after the other.
After the duelling was over, Mahan the Commander-in-Chief of the Byzantine forces asked his forces to launch the assault. The Muslims withstood their ground. At sunset when the action ended there were more casualties on the Byzantine side than on the side of the Muslims.
On the second day, the Muslims were still at morning prayer when the Byzantines launched the attack. The Muslims got into position immediately and the two armies clashed. The Byzantines did not press at the Muslim centre; they directed their pressure on the Muslim flanks. The Muslim right was led by Amr bin Aas. The Muslim corps on the right withstood two attacks, but at the third attack which was very severe they fell back in some disorder. The Muslim cavalry held up the Byzantine advance for some time, but they were unable to hold it for long. Repulsed by the Byzantines the Muslims fell back on their camp. Here they were greeted by Muslim women with stinging rebukes. That made the Muslim warriors turn back from the camp. The Muslims launched a counter attack and the Byzantines were pushed back.
The Muslim left flank was led by Yazeed. The corps under Yazeed withstood the first attack but fell back under the severity of the second attack. The Muslim cavalry launched the counter attack but it was repulsed, and the Muslims fell back to their camp. Here the Muslim women put the Muslim warriors to shame. They exhorted them to return to battle and show their courage. They returned to the battle and launched a coupler attack.
Seeing the pressure on the flanks, Khalid decided to come to their help. First he turned to the right wing and struck at the flank of the Byzantine army. The Byzantines reeled under the pressure of these blows and beat a retreat. Thereupon the corps of Amr regained all the ground they had lost.
Khalid next turned to the left wing, and attacked the Byzantine corps. Here again the Byzantines withdrew under the force of the counter attacks launched by the Muslims from the front as well as the flank. The attack on the Muslim side was led by Zarrar. He killed Derjan, the Commander of the Byzantine corps.
By sunset the two flanking armies of the Byzantines had been pushed back. The Muslims had faced a critical situation, but they had managed to regain the lost ground. When the battle ended on the second day the result was still undecisive.