The ongoing war against the Banu Thalaba tribe was mainly as a result that they had killed nine Muslims preceding this incident. Hence, the Prophet (p) sent out his Companions to deal with those who were involved.
Kitab al-tabaqat al-kabir – Ibn Sa’d
“THE SARIYYAH OF ZAYD IBN HARITHAH TOWARDS AL-TARAF
… The Apostle of Allah sent Zayd Ibn Harithah to al-Taraf, which is a spring adjoining al-Mirad before al-Nukhayl. It is a distance of thirty six miles from al-Madinah on route leading to al-Mahajjah through al-Baqarah. He set out with fifteen persons (to meet) Banu Tha’labah. …” 
“In this year a fiteen-man raiding party led by Zayd b. Harithah went al-Taraf in Jumada II against the Banu Tha’labah. The bediouns fled, fearing that the Messenger of God had set out against them. …” 
Imam Zaid Shakir:
“Not Only did the Prophet love Zayd b. Haritha, he trusted him with the command of many of the expeditionary forces that took the field in the early days of Islam. In fact, Zayd b. Haritha was the most prominent Muslim commander during that time. He led the expedition at al-Qarada in the Najd, to Bani Sulaym at al-Jamum, to Al-Is, to al-Taraf…” 
This tribe have previously attacked and killed Muslims:
1. Expedition Of Dhat Al-Riqa?
2. First Raid on Banu Thalabah?
3. Invasion of Thi Amr?
Don’t forget to follow Discover The Truth on Facebook and Twitter. PLEASE help spread the word by sharing our articles on your favourite social networks. Thank You!
 Kitab al-tabaqat al-kabir, by Ibn Sa’d, volume 2, page 108
 History of al-Tabari: The: The Victory of Islam: Muhammad at Medina, volume 8, page 94
 Scattered Pictures: Reflections of an American Muslim : an Anthology of Essays [2005, Zaytuna institute] By Imam Zaid Shakir, page 72