The garrison at Tayma. When active operations were being undertaken in Iraq, Abu Bakr stationed a garrison at Tayma to the east of Tabuk to protect the border against any attack by the Byzantines from Syria. The garrison at Tayma was commanded by Khalid bin Saeed.
Discomfiture of Khalid bin Saeed. The Muslims won spectacular success on the Iraq front. This created in Khalid bin Saeed the urge to score some victory on the Syrian front as well. Early in 634 C.E., Khalid bin Saeed sought the permission of Abu Bakr to advance into Syria. Abu Bakr permitted Khalid bin Saeed to enter into Syria, but he was directed that the operations should be undertaken as a reconnaissance measure only, and no attempt should be made to get involved in any serious hostilities with the Byzantines.
Khalid bin Saeed advanced into Syria, and the Byzantine forces retreated before him. That gave Khalid bin Saeed the impression that the victory of Syria would be a walk over, and that he could win laurels on the Syrian front as his namesake had won on the Iraq front. Khalid bin Saeed accordingly penetrated deep into Syria fin pursuit of the Byzantine forces. When Khalid bin Saeed was cut off from his base, the Byzantines enveloped the Muslim forces and launched a vigorous counter attack. In this encounter, the Muslims suffered a serious defeat. Khalid bin Saeed lost his son in action and that unnerved him. In a state of desperateness, he escaped from the battlefield. The command was thereafter assumed by Ikrama bin Abu Jahl, who retrieved the position by evacuating the Muslim forces. Abu Bakr felt annoyed at the discomfiture of Khalid bin Saeed, and directed him not to come to Madina. Khalid bin Saeed accordingly retired to the interior of the desert at some distance from Madina.
Jihad on the Syrian front. On return from the pilgrimage in February 634 C.E., Abu Bakr issued a call to arms for Jihad on the Syrian front. In response to the call, tribal contingents came over to Madina from all parts of Arabia. By March 634 C.E., a large force assembled at Madina ready to march to Syria. Abu Bakr organized all these warriors into four corps, each comprising of 7,000 men.
The first corps was placed under the command of 'Amr bin Al Aas. It was required to advance to Palestine via Eila and the valley of Araba.
The second corps was placed under the command of Yazeed bin Abi Sufyan. It was directed to proceed to Damascus via Tabuk.
The third corps under Shurahbil bin Hasana was required to proceed to Jordan.
The fourth corps under Abu Ubaida bin Al Jarrah was required to advance to Emessa. All the columns were required to act independently, if the forces were to integrate, Abu Ubaida was to be the Commander-in-Chief.
Abu Bakr's address to the Muslim forces. The Muslim forces marched from Madina in the first week of April 634 C.E. The corps led by Yazeed bin Abi Sufyan was the first to leave. Thereafter, other corps left according to program. Abu Bakr addressed the forces at the time of their departure in the following terms: "In your march be not hard on yourself or your army. Be not harsh with your men or your officers whom you should consult in all matters. Be just and abjure evil and tyranny, for no people, who are unjust, can prosper or achieve victory over their enemy. When you meet the enemy turn not your backs except to maneuver for the battle or to re-group, for he, who does so, earns, the wrath of Allah. His abode will be hell, and what a terrible place it is! And when you have won a victory over your enemies, kill not women or children or the aged. Do not slaughter any beasts except for eating. And break not the pacts that you make with other people. You will come upon persons who live like hermits in monasteries, believing that they have given up all for God. Let them be as they are, and do not harm their monasteries. You will meet other persons who are partisans of Satan and worshippers of the Cross who shave the center of their heads so that you can see the scalp. Assail them with your swords, until they submit to Islam or pay the Jizya. In all transactions fear God, and when in difficulty invoke His aid. Now depart in the name of God. May He protect you."
Yazeed and his corps sped on the road to Tabuk. The corps of Amr bin Al Aas took the route to Eila. Then followed the corps of Shurahbil, and next came the corps of Abu Ubaida, each a day's march from the other.
Encounter with the Byzantines. At the border, the corps of Yazeed struck against a force of the Christian Arabs sent forward by the Byzantines as a reconnaissance force. The Christian Armies withdrew and Yazeed marched to the valley of Araba. The corps of 'Amr bin Al Aas reached Eila. Both the corps fought against Byzantine detachments sent to intercept their advance. The Byzantine detachments suffered defeat, and had to retreat after suffering considerable loss. In the meantime, the corps of Shurahbil and Abu Ubaida reached the region Basra and Jabiya.
Plan of the Byzantines. The Byzantine emperor, Heraclius now planned ac ion on a large scale. He mustered forces at Ajnadeen numbering over one hundred thousand. The position became critical for the Muslims for the four small corps that had penetrated into Syria were no match for such a large concentration of the Byzantines. Abu Ubaida wrote to Abu Bakr asking for reinforcement, and Abu Bakr decided to send Khalid bin Walid from Iraq to Syria.