Some of have claimed that the invasion of Bahran was carried out for no reason. If we were to read historical evidences for this incident, it becomes quite clear that the enemy Banu Sulaym were assembling an army to launch an attack against the Muslims. This in turn made the Muslims prepare and engage the enemy.
Kitab al-tabaqat al-kabir – Ibn Sa’d:
GHAZWAH OF THE APOSTLE AGAINST BANU SULAYM
Then (occurred) the ghazwah of the Apostle of Allah, against Banu Sulaym at Buhran, on 6 Jumada al-Ula after the commencement of the twenty-seventh month from his hijrah.
Buhran lies in the territory of al-Furu, and the distance between al-Furu and al-Madinah is eight burds.
The cause was that it (report) had reached him (Prophet) that a large party of Banu Sulaym had mobilized there.
He set out out with three hundred men from the Companions. He appointed Ibn al-Maktum as his vicegerent in al-Madinah. He marched rapidly until he arrived at Buhran. He found that they had dispersed and returned to their springs. He (Prophet) returned, and there was no encounter. His absence (from al-Madinah) was for ten nights. 
Hajjah Amina Adil:
THE RAID OF BAHRAN
After the Raid of Dhu Amarr, the raid of Bahran took place against the Bani Sulaym. The reason was this: news was received that in the area of al-Furu a number of unbelievers of the Bani Sulaym were assembling against the Muslims and getting ready to launch an attack.
Thereupon Muhammad set out with three hundred men to forestall the attackers. As soon as the tribes heard of the advance of the Holy Prophet’s forces, they dispersed and fled into the hills. Muhammad returned to Madinah, having remained for twelve nights outside of the city. 
Shaikh Abdullah Muhammad:
THE RAID OF BAHRAN
It is also known as the Banu Sulaim Expedition, in the area of Al-Fara. The Cause of it was that the Messenger of Allah had been informed that a large number of men from Banu Sulaim had gathered there, so he set out at the head of three hundred men from among his Companions and found that they had dispersed among their watering-holes, so he returned without encountering any fighting.
During his absence, he had appointed Ibn Umm Maktoom to take care of the affairs of Al-Madinah, according to Ibn Hisham. He was absent for ten night, and it was said that he set out in search of the Quraish; this was reported by Ibn Ishaq. 
Bashir Ahmad M.A.;
Ghazwah of Buḥran – Rabi‘ul-Awwal 3 A.H.
The preparations underway for the two attacks to be made by the Banu Sulaim and Banu Ghaṭafan have already been described previously. It has also been mentioned as to how by the Grace of God, the instant and timely strategy of the Holy Prophet protected the Muslims from the evil of these blood-thirsty tribes. However, how can one in whose heart the fire of enmity burns sit at rest.
Much time had not passed since the Ghazwah of Dhi ‘Amr, that is to say, in late Rabi‘ul-Awwal 3 A.H.,2 the Holy Prophet received the horrific news that the Banu Sulaim were once again regrouping in a very large number at Buḥran, with the intention of launching a sudden attack upon Madinah, and that a party of the Quraish was also accompanying them. With no other choice, the Holy Prophet set out from Madinah once again with a group of Companions.
However, as was their habit, these wild beasts of Arabia, who lay in ambush to strike their prey suddenly and in a state of inattention, dispersed upon receiving news of the imminent arrival of the Holy Prophet. After a stay of a few days, the Holy Prophet returned.
The fact that the Banu Sulaim and Banu Ghaṭafan would gather again and again with the intention of launching a sudden attack upon Madinah clearly demonstrated that these barbaric and warlike tribes of the Arabian desert were very deadly enemies of Islam.
Day and night, they would remain preoccupied in order to find some opportunity by which the Muslims would be utterly destroyed. Just attempt to visualize the vulnerable state of the Muslims at the time, as to how their days were passing in that era.
On the one hand, there were the Quraish of Makkah who had become blinded due to their enmity of Islam and due to their spirit of revenge for the Battle of Badr.
Clinging to the drapes of the Ka‘bah, they took vows that they would not rest until the Muslims had been annihilated. On the other hand were these blood-thirsty wild beasts of the Arabian desert, who were restless to drink the blood of the Muslims due to their being incited by the Quraish and their own animosity towards Islam.
As such, take note of how many times within the course of a few months after Badr, the Holy Prophet was compelled to travel personally in order to safeguard himself from the deadly motives of the barbaric tribes of Arabia. As Sir William Muir has described, these were days of scorching heat as well, and not to mention, this heat was of the Arabian desert.
If it were not for the special succour of God, and if the vigilance of the Holy Prophet had not kept the Muslims constantly watchful and alert, and if the Holy Prophet had not employed strategies to scatter the force of the enemy prior to their launching a sudden attack at night, the Muslims would have surely been destroyed and ruined in those days; and these were only external threats.
As far as internal threats were concerned, they were no less either. Even in Madinah itself, there existed a group known as the hypocrites, who lived among the Muslims as their own, and it would definitely be no exaggeration to refer to them as a snake in the grass.
In addition to them were the treacherous and habitual conspirators in the form of the Jewish people, whose enmity had reached the furthest limits in its depth and breadth. Goodness Gracious! What a time of adversity this was for the Muslims!!
Let us hear it in their own words. Ubayy bin Ka‘b, a renowned Companion relates: Meaning, In that era, the state of the Companions was such that they would not even put off their arms at night and during the day they would walk around during the day armed in case of a sudden attack.
They would say to each other let us see if we live till such a time when we might be able to sleep in peace and security at night without any fear except the fear of God.” What difficulty and helplessness, and what a longing for a life of peace and security is hidden in these words. Every just individual can measure this for himself. 
 Kitab al-tabaqat al-kabir – Ibn Sa’d, volume 2, page 41
 Muhammad, the Messenger of Islam: His Life & Prophecy by Hajjah Amina Adil, page 313
 Biography Of The Prophet – By Shaikh Abdullah Muhammad Ibn Abdul Wahab, volume 1, page 473
 The Life & Character of the Seal of Prophets – (Sirat Khatamun-Nabiyyin) By Bashir Aḥmad M.A., volume 2, page 294 – 296