Two things happened in which Prophet Muhammed (p) made a decision to deal with those hostile enemies. (1) In the territory of Dumatul Jandal there were highway robberies. People were getting robbed. (2) In the same area there was an army preparing to attack Madinah.
With the impending army and robberies committed in that area, the Prophet (p) decided to engage these criminals.
Historical reports on this incident
“THE EXPEDITION TO AL-JANDAL AND OTHER EVENTS
According to al-Waqidi: In this year he mounted an expedition against Dumat al-Jandal in the month of Rabi’I. The reason for it was that word reached the Messenger of God that a host had assembled there and had approached his territories; so the Messenger of God mounted an expedition against them and reached Dumat al-Jandal, but he had not clash with the enemy. He left Siba b. Urfutah al-Ghifari in charge of Medina.” 
Kitab al-tabaqat al-Kabir – Ibn Sa’d:
“GHAZWAH OF THE APOSTLE OF ALLAH, TO DUMAT AL-JANDAL
Then (occurred) the ghazwah of the Apostle of Allah, to Dumat al-Jandal in Rabi al-Awwal, after the commencement of the forty-ninth month from his Hijrah. They (narrators) said: (The news) reached the Apostle of Allah, that a large number of men assembled at Dumat al-Jandal and that they treated cruelly the camel-riders when they passed by them, and intended to attack al-Madinah. It (dumat al-jandal) lies on the border of Syria at five days journey from Damascus and fifteen or sixteen days journey from al-Madinah. The Apostle of Allah, called the people and appointed Siba Ibn Urfutah al-Ghifari as his vicegerent at al-Madinah and set out on 25 Rabi al-Awwal with one thousand Muslims. He travelled during nights concealing himself in the days. He had for his guide a person of Banu Udhrah who was known as Madhkur. When he reached there, they were evacuating their habitations, but there were the traces of goats and sheep. He attacked their … herdsmen and he captured whom he could, and those who could, escaped. The news reached the inhabitants of Dumah who dispersed. The Apostle of Allah, alighted in the plain and found none. He stayed there for several days and sent expeditions which returned without having come across any one. …” 
Scholars commenting on this incident
“The news filtered into Madinah that the tribes around Dumat al Jandal near Syria were waylaying passers-by and plundering their belongings. They had become so arrogant that they began thinking of attacking Madinah, and a great horde gathered to make this raid. The Prophet thus marched with a thousand men, resting during the day and travelling during the night surprised the enemy. The distance between Yathrib and Dumat al-Jandal was fifteen days, which the Muslims traversed with the help of a skilled guide. When they reached the camp of their enemy, they swept down upon it in a surprise attack and the hordes fled helter-skelter. …” 
Shaykh Allamah Shibli Nomani:
“GHAZWA DUMAT AL-JANDAL
In the month of Rabi al-Awwal of the same year, the Prophet (p) was informed of the disbelievers mobilising a huge army at a place known as Dumat al-Jandal. He marched out with 1,000 companions, and again the enemy took to flight on hearing his approach.” 
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 The History of al-Tabari -The Victory of Islam: Muhammad at Medina A.D., volume 8, page 4 – 5
 Kitab al-tabaqat al-kabir – by Ibn Sa’d volume 2, Page 76
 Muhammed Al-Ghazali, Fiqh-us-seerah, page 312
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – Volume 2, page 99