After the Byzantines had lost Damascus, the emperor Heraclius planned a large scale action against the Muslims. His strategy was to cut off the Muslim forces in Syria from communication with Arabia. With this object, he ordered a large concentration of the Byzantine force at Beisan to the west of the Jordan river to the south of Damascus.
The Muslims had only a small garrison at Fahl to the east of Jordan at some distance from Beisan. When Abu Ubaida came to know of the concentration of the Byzantine force at Beisan, he held a council of war. The concensus of opinion was that all the forces that the Muslims could muster should march to Fahl, and meet the Byzantine force before it could gather further strength. Abu Ubaida left a corps under the command of Yazeed at Damascus, and the rest of the Muslim forces marched to Fahl.
When the Byzantines came to know that the Muslims were marching southward they dammed the Jordan river, and thereby flooded the countryside around Fahl. The Muslim forces cantoned at Fahl. The Byzantine forces were led by Saqlar bin Makhraq. He asked the Commander of the Muslim forces to depute some representative for the purposes of negotiation Abu Ubaida deputed Muadh b. Jabal as the Muslim representative.
The Byzantines had a cloth of gold laid for Muadh to sit. Muadh, however, sat on the bare ground. When asked to explain his conduct he said:
"It is the wont of slaves to sit on bare ground. I am the slave of God, and therefore I sit on the bare ground."
Addressing Muadh Saqlar advised the Muslims to attack Persia and Abyssinia where the chances of their success were greater. He said that in the case of the Byzantines, the Muslims were ill advised to wage war, for the Byzantine could muster forces as numerous as the stars in heaven.
Muadh said that they would launch a campaign against Persia in due course. He said that the Muslims were in no way afraid of the large strength of the Byzantine forces. They were fighting in the way of Allah and they were fortified with the faith that God would help them. Saqlar said that the Muslims could have Baqla and some other districts adjoining Arabia provided they withdrew from Syria. Muadh turned down the offer. He offered the Byzantines the usual three alternatives. Muadh then returned to the Muslim camp.
The following day a Byzantine representative came to the Muslim camp. He found the Muslim Commander-in-Chief Abu Ubaida dressed as an ordinary soldier sitting on the bare ground examining arrows. He gave Abu Ubaida a message from Saqlar that if the Muslims withdrew from Syria he would pay them a good deal of money. Abu Ubaida rejected the offer, and said that the issue between the Muslims and the Byzantines would be decided on the battle-field.
The following day the Muslims decided to cross the river, and attack Beisan. Khalid led the advance guard. The Muslim forces had not proceeded very far when they got stuck in the mud, and had great difficulty in extricating themselves. They accordingly returned to Fahl and decided to wait.
The Byzantines were happy that their stratagem of flooding countryside had paid dividends. Byzantine had guides who assured them that they could negotiate the marsh. The Byzantine forces commanded by Saqlar crossed the Jordan river and proceeded to Fahl. They hoped to catch the Muslims unaware.
The Byzantines launched the attack on 23rd January 635. As the Byzantines advanced, all advantages lay with them. They were larger in strength and they were better equipped. The topography was also in their favour. They could negotiate the marsh. They opend the atttack with a rain of arrows. The Muslim cavalry was led by Khalid and they formed the Muslim vanguard. Due to the rain of arrows from the Byzantine side the Muslim forces had to fall back. They steadily withdrew until they were on firm ground beyond the flooded area. Then the Muslims charged. In the hand to hand fight that ensued the Muslims were superior to the Byzantines. The Commander-in-chief of the Byzantine forces Saqlar and many other commanders were killed. That demoralised the Byzantines. Overpowered the Byzantine forces pulled back and decided to withdraw to Beisan. The Muslims increased their pressure. Under the pressure of the Muslim assault the retreat of the Byzantines soon became a rout. The Muslims played havoc with the forces. The retreating Byzantine got bogged up in the mud, and the pursuing Muslims made mince meat of them. The marsh which the Byzantines had created to trap the Muslims became a death trap for the Byzantines themselves. Over ten thousand Byzantines perished in the battle of Fahl. The marsh came to be studded with the dead bodies of the Byzantine soldiers. The battle ended in victory for the Muslims. Because of the mud, the battle of Fahl came to be known in the Arab chronicles as the Battle of the Mud.
After the battle of Fahl, the main Muslim army under Abu Ubaida and Khalid returned to Damascus. One contingent was left to conquer Beisan. Another contingent proceeded to capture Tabariyya.
The Muslims crossing the Jordan proceeded to Beisan. The Persians shut the gates of the city in the face of the Muslims, and the Muslims laid siege to the city. After a few days finding resistance futile the Byzantines surrendered and agreed to pay Jizya.
Tabariyya was eighteen miles from Beisan. It was the chief town of Jordan. The town was fortified and at the approach of the Muslims, the gates of the city were shut against them. The Muslims laid siege to the town, and blocked all routes to the town. After the fall of Beisan, the citizens of Tabariyya also found that any further resistance was useless. They, therefore, surrendered and agreed to pay Jizya. They vacated fifty per cent of the houses in the city which were occupied by the Muslims. With the fall of Tabariyya, the whole of Jordan came under the occupation of the Muslims. The campaigns in Jordan ended in February 635 and the Muslims settled down to administer the land.