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.