وَمَآ أَصَٰبَكُمْ يَوْمَ ٱلْتَقَى ٱلْجَمْعَانِ فَبِإِذْنِ ٱللَّهِ وَلِيَعْلَمَ ٱلْمُؤْمِنِينَ
Commentary:
The "meeting of the parties" refers to the Battle of Uhud which is under review here.
The clause لیعلم المؤمنین rendered as, that He might distinguish the believers, does not mean, "that He might know the believers", as generally translated. According to Quranic idiom, the expression simply means that Allah might mark out, or make known, or distinguish believers from disbelievers. See also note on 3:141.