After assuming the Caliphate the first issue that Abu Bakr was called upon to decide was whether the expedition to Syria which the Holy Prophet had directed to be sent under the command of Usamah should proceed to its destination, or should the expedition in view of the change in circumstances be abandoned.
The background of the expedition was that in 639 A D. the Holy Prophet had sent an expedition against the Syrians under Zaid bin Harith. In the confrontation that had taken place at Mutah, Zaid had been martyred. The command had thereafter been taken over by Jafar bin Abu Talib and he had also been martyred. Abdullah bin Rawaha who had next taken the command had also been martyred.
At that critical stage, Khalid bin Walid had taken over the command By his skillful tactics and superb strategy he had succeeded in retrieving the position and bringing back the Muslim forces safely to Madina. For this act ot heroism, Khalid bin Walid had received from the Ho]y Prophet the title of'Saifullah'-the Sword of Allah.
In 632 A.D. on return from the `Fafewell Pilgrimage', the Holy Prophet ordered a detachment to be sent against the Syrians under the command of Usamah the son of Zaid bin Harith. Some persons objected to the command of Usamah, a mere youth of twenty when other veteran commanders were available. The Holy Prophet overruled the objection, and declared that Usamah was worthy of the command.
When the Holy Prophet fell ill the detachment of Usamah was camped at Jorf a few miles from Madina on the road to Syria. On account of the serious illness of the Holy Prophet, Usamah delayed his departure. When the Holy Prophet died, Usamah returned to Madina, and sought further orders from the new Caliph.
Most of the Companions were of the view that at that critical stage in the history of Islam when most of the tribes had apostatised from Islam, and Madina itself was surrounded by hostile tribes it was dangerous to send the army outside the country. They were further of the view that if the expedition was necessarily to be undertaken, there should be a change in the command and some veteran soldier shonld be appointed as the commander instead of Usamah. The companions chose Umar as their spokesman to represent their view point before Abu Bakr.
Umar saw Abu Bakr, and represented the case with considerable vehemence. As regards the issue whether the expedition should or should not be undertaken Abu Bakr said that as the Holy Prophet had insisted on sending the expedition, it would be a breach of faith on his part to reverse the orders of the Holy Prophet. Umar tried to argue that if the army was sent, the city of Madina would be exposed to attack by the enemy, and the Caliphate itself would be in danger. To this Abu Bakr replied:
"Who am I to withhold the army that the Holy Prophet had ordained to proceed? Come what may, let Madina stand or fall, the Caliphate live or perish, the command of the Holy Prophet shall be carried out."
As regards the issue about the change of command Abu Bakr said:
"This objection had been raised before the Holy Piophet as well and he had rejected the objection. How can I as the successor of the Holy Prophet accept an objection which the Holy Prophet had in his wisdom rejected?"
"O the Caliph of the Holy Prophet, you are wiser than us all. You are right. May God bless you and your decisions".
Thereafter Umar explained to the companions the decisions of Abu Bakr, and the justification therefor.
The army under Usamah was accordingly directed to proceed to its appointed task. On the eve of the departure of the army, Abu Bakr addressed the soldiers and gave them instructions regarding their conduct and responsibilities. Umar was also included among the soldiers in the army of Usainah. Turning to Usamah, Abu Bakr said:
"I beg one favour of you. Do not take Umar with you. Leave him here to help me."
The army of Usamah marched from Jorf to Syria. Umar was left at Madina to serve as an Adviser to Abu Bakr.