Reason of the Battle

we have already spoken about al-‘ushairah invasion when a caravan belonging to quraish had escaped an imminent military encounter with the prophet [pbuh] and his men. when their return from syria approached, the prophet [pbuh] despatched talhah bin ‘ubaidullâh and sa‘id bin zaid northward to scout around for any movements of this sort. the two scouts stayed at al-hawra’ for some days until abu sufyan, the leader of the caravan, passed by them. the two men hurried back to madinah and reported to the prophet [pbuh] their findings. great wealth amounting to 50 thousand gold dinars guarded by 40 men moving relatively close to madinah constituted a tempting target for the muslim military, and provided a potentially heavy economic, political and military strike that was bound to shake the entire structure of the makkan polytheists.

the prophet [pbuh] immediately exhorted the muslims to rush out and waylay the caravan to make up for their property and wealth they were forced to give up in makkah. he did not give orders binding to everyone, but rather gave them full liberty to go out or stay back, thinking that it would be just an errand on a small scale.