As Salamu Alaikum ,

Jazakumullahu Khayr sisters for your responses. Yes it was Abu Bakr RA. I will go into it with more details.

Prophet {Sallal lahu alayhi wasallam} was aware of threat from Roman army. He made a force ready to combat them, in which he recruited Abu Bakr, Umar RA and many other leading companions. But he wisely appointer Usama bin Zaid RA as commander in chief of that army. Besides being courageous Usama was spurred on by the fact that his father Zaid ibn Harith RA was killed by the army of Romans in the battle of Muta. This army was unable to advance, as Prophet {saws} was ill and then passed away

After Abu Bakr was appointed as khalif, he had many challenged before him.  But the first task he undertook was to adavance this army, the job which cud not be accomplished during the time of Prophet. Moreover the situations were so tough then, as Madinah was liable to be attacked by arab tribes anytime. There were revolts within arab, because of apostates and those tribes who looked upon zakat at burden. These rebels were trying to attack Medinah

So the companions, felt that the army shud stay within and defend Madinah. The other reason was as Sis Alwardah said,  the army was n't accepting Usama RA as there commander. The reason being, he was just 17 years old.

Abu Bakr RA, listened to all these and said, " Even if I am the only one left to be devoured by wild beasts , still i cannot recall an army, which Prophet {saws} himself dispatched.. He dismissed the matter of Usama RA's youth Madinah may stand or fall, Caliphate may come or go,  the Prophet's word must be fulfilled". 

He himself went on foot to send the army on its way under the commandership of Usama RA. With Usama aloft on his mount, the Caliph of the muslims walked alongside , speaking with him on matters concerning the militray campaign. Usama requested Abu Bakr RA, would agree to neither. He wanted to put an end  to the muslim's misgivings about the leadership of Usama RA's leadership, and this was the most practical and effective way of doing it. Their reservations vanished on seeing the Caliph walking alongside Usama's mount.

Subhan Allah!

As the news of the advance of Usama's army spread around Arabia, opponents saw in it a sign of Muslim's confidence. They presumed that the Prophet's followers must have considerable reserves of strength to be able to send an army so far from Madina at such a critical time. They decided to await the outcome of the Syrian campaign before attacking the city ; if the muslims met defeat, then they would be sufficiently weakened for an offensive against their capital to be feasible.

Usama RA was eminently successful against the Romans, in the campaign which lasted for 40 days. It proved thta Usama RA was the most suitable person for this expedition for, his father having being martyred fighting the Roman army at Muta, he was keen to revenge. A large number of captives and great quantities of booty  went back with the muslims to Medina. The rebels of arab,  lost hearts on seeing this, and their revolt  was quelled with comparitive ease. So it was that the muslims achieved success on both fronts, simply by having done as the Prophet said.