Before the Prophet’s (p) demise, he sent Usama Bin Zayd expedition to Syria. There were few reasons the Prophet (p) had to send his companions to Syria.
1. Syria which was under the control of the Byzantine (Roman) Empire, they were forcing people not to embrace Islam. Religious freedom was out of the window.
2. Byzantine (Roman) Empire were also taking over territories in the South which was the land of the Muslims.
3. A report in Ibn Asaakir (which will be mentioned at the end) hints to us that they were mobilizing troops to attack the Muslim community.
This was not the first time the Byzantine were hostile and attempted to kill Muslims. The battle of Tabuk took place precisely as a result of Byzantine were gathering troops to attack the Muslim community. Even before the battle of Tabuk, these people murdered an innocent messenger who was bringing a letter to the king of Busra. As a result of the killing, the battle of Mu’tah took place. It was in this same place (Mutah) where Zayd’s father was slaughtered.
With all the above in blessed Prophet’s (p) mind, he had to come up with something to give people freedom to choose whatever religion they want to follow and stop the Byzantine’s from persecuting innocent people. Hence, the Prophet (p) sent his companions under the leadership of Usama Ibn Zayd (ra) to deal with them.
“Abu Ja’far (al-Tabari): I have received an account on the authority of Ibn Humayd – Salamah – Muhammad b. Ishaq – Abd al-Rahman b. al-Harith b. Ayyash b. Abi Rabi’ah: In the year 11/632, during the month of Muharram, the Messenger of God ordered the people to undertake an expedition to Syria. He put Usamah, the son of … Zayd b. al-Harithah, in command over them, ordering him to lead the cavalry into the territory of al-Balqa and al-Darum in the land of Palestine. The People got ready and the first emigrants went forth all together with him. As the people were preparing for the expedition, the Prophet began to suffer from the sickness by which God took him to what honor and compassion He intended for him. [It took place] toward the end of Safar or at the beginning of Rabi’i.” 
Dr. Mustafa As-Sibaa’ie:
“THE SENDING OF USAAMAH TO PALESTINE
“The last thing that the Messenger of Allah did to spread and protect the call of Islam, and to repel the aggression of those who wanted to attack the new state and put an end to it was to prepare an army to go to Syria under the leadership of Usaamah Ibn Zayd. He commanded him to take his army to the border of Al-Balqa and Ad-Daroom in the land of Palestine. This army was composed of both Muhaajireen and Ansaar, as well as Muslims from the areas around Madeenah, and no one stayed behind. When the army was outside Madeenah, preparing to set out, the Messenger of Allah fell ill with what was to be his final illness. …” 
“EXPEDITION TO PALESTINE
The prophet returned to Madinah, aware that he was near the end of his mission, and he spent most of his time praising and glorifying Allah who had given him success after success during his twenty-three years of Prophethood. People were entering Islam in multitudes, and delegations continued to arrive before him.
In Rabi Al-Awwal of 11 A.H., the Prophet sent Usama bin Zayd with seven hundred soldiers to the territory of Balqa and Darum in Palestine. They were to stage a show of might against the Romans, who had resumed their hostile acts. The army set out and at Jarf, only three miles outside of Madinah, they received news that the Prophet was very ill. They encamped there awaiting further news of the Prophet’s health. With the Prophet’s subsequent death, Usama and his men went on with the Prophet’s subsequent death, usama and his men went on with their expedition and became the first people to lead a military expedition during the Caliphate of Abu Bakr Siddeeq.” 
Shaykh Muhammad Al-Ghazali:
“Within a few days the Prophet’s death Islam became entangled in a fierce struggle with paganism, which suddenly came to life, and Christianity, which controlled the north of the peninsula, prevented anyone from embracing Islam and stopped its spread by force. Te desert had not seen the like of these violent battles during the lifetime of the Prophet himself.” 
Dr. Ali Muhammad As-Sallaabee:
FIRST: BASED ON THE PROPHET’S WISHES, ABU BAKR SENDS OUT USAAMAH’S ARMY
During the lifetime of the Prophet there were two major empires that bordered the Arabian Peninsula: The Roman Empire and the Persian Empire. As for the former, it controlled large areas of land in the northern part of the Peninsula. The rulers of those areas acted as viceroys for the Roman emperor: they were appointed by the Empire and were completely under the Empire’s control.
The Romans had a very low opinion of the Arabs who lived in the Peninsula; they didn’t even deem it worth their while to seize control of the entire peninsula. So they were somewhat annoyed when they heard what was taking place: Arabs were getting organized under the banner of the same religion. Wanting to teach a lesson to what they considered to be an upstart nation, the Romans made certain incursions towards the south.
Meanwhile, the Prophet sent messengers towards the northern areas of the peninsula, areas that were collectively known as the region of Ash-sham (Syria and surrounding regions). For example, he sent Dahiyyah Al-Kalbee with a letter to Haqaql, the emperor of Rome; in it, the Prophet invited Haraql to embrace Islam. To his own discredit, Haraql turned away from the truth, refusing to accept that invitation. …” 
This following narration gives us information that the enemies were indeed mustering troops to attack the Muslims. When usama Ibn Zayd (ra) sent over one of his spy’s to monitor what the enemy was doing. He reported back stating that the enemy’s army didn’t even muster their own army yet [Ibn Asaakir]:
“Usaama set off speedily and passed through peaceful areas where the people had not forsaken Islam, such as the areas of the Juhayna clan and ither tribes belonging to the Qudaa’ah tribe. When he reached Waadi Qura, Usaama sent a spy ahead from the Banu Udhra tribe, whose name was Hurath. Taken to his mount, he rode ahead of Usaama until he reached Ubna (their intended destination). He surveyed the area and searched for the best route (for the army to take). He then returned quickly and enjoined Usaama at a place that lay a stance of two night’s journey from Ubna. He informed Usaama that the people were oblivious of the (Muslim) army and had not even mustered their own army. Usaama then ordered the Muslim army to move speedily and attack the enemy unawares before they had a chance to muster their forces.” (Ibn Asaakir volume 1, page 130) 
These crucial wordings used in this report indicates that the Muslims did receive information prior that the Byzantine were preparing to attack the Muslims. Hence, Usaama got to them before they had the chance of attacking the Muslims.
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 The History of al-Tabari: The Last Years of the Prophet: the Formation of the State A.D. 630-632/A.H. 8-11:, volume 9, Page 163 – 164
 The Life of Prophet Muhammad: Highlights and Lessons [Transated Nasiruddin al-Khattab – Copyright International Islamic Publishing House, 2003, King Fahd National Library] By Dr. Mustafa As-Sibaa’ie, page 184
 When The Moon Split By Safi-ur-Rahman Al-Mubarakpuri, page 303
 Fiqh-us-seerah: Understanding the life of prophet Muhammad By Muhammad Al-Ghazali, page 502
 The Biography of Abu Bakr As Siddeeq, by Dr. Ali Muhammad As-Sallaabee, Page 313-314
 Ibn Asaakir volume 1, page 130, the narration also appears in Mukhtasar Ibn Asaakir. Kanzul Ummaal volume 5, page 312 and Fat’hul Baari volume 8, page 107 [Hayatus Sahabah – By Shaykh Muhammad Yusuf Kandhelvi (r.a), volume 1, page 419]