Some critics have claimed that the sole purpose of Dahhak al-Kalbi’s expedition was to force the Banu Kilab tribe to embrace Islam. There is no truth to this.
The reason companions of the Prophet (p) were despatched was for missionary purposes i.e., preaching the message of Islam. They did exactly that in a peaceful way, but the Banu Kilab instead of just leaving the Muslims to preach as they wished, they started attacking the Muslims for merely propagating the message of Islam. Hence, a battle ensued.
Dr. Shawqi Abu Khalil:
“THE EXPEDITION OF AD-DHHAK IBN SUFYAN AL-KILABI TO BANU KILAB IN RABUL-AWWAL 9 AH.
Al-Dahhak and the men with him marched on Banu Kilab and met them in Az-zujj (Zujj Lawah), were fighting took place.” 
“The mission of Dahhak bin Sufyan Al-Kilabi to Bani Kilab in Rabi Al-Awwal in the year 9 A.H.
This mission was sent to Bani Kilab to call to embrace Islam. Refusing to embrace Islam, they started to fight against the Muslims, but were defeated and sustained one man killed.” 
“DESPATCHING PREACHERS – E.G.
In the year 9AH, Dahhak bin Sufyan al-Kilabi went to Bani Kilab to call them to embrace Islam. Refusing to embrace Islaam, they started to fight the Muslims, but were defeated.” 
 Atlas Al-sirah Al-Nabawiyah – Atlas On The Prophet’sBiography Places, Nations, Landmarks, By Dr. Shawqi Abu Khalil, page 235
 Sealed Nectar (‘Ar-Raheeq Al-Makhtum’), By Safiur-Rahman Al-Mubarakpuri, page 187
 Notes On Entering Deen Completely: Success, Ummah, Renewal By Talib Jalee, page 512