You were on the near side of the valley and the infidels on the far side, and the caravan below you marching by; a web of circumstance calculated and timely arranged by the Omnipotent. Had you tried to plan such a precise arrangement you would have failed. Allah had planned it in such a manner as to serve His purpose and to help you obtain an advantage over your enemy who outnumbered you to a great disproportion, the survivors among the infidels will then realize that their defeat was made by Allah's Omnipotence and Authority and the survivors among the Muslim victors realize Allah's Omnipotence and Sovereignty and Allah is indeed Sami'un with unlimited audition and 'Alimun.