﴿ وَمَآ أَصَٰبَكُمْ يَوْمَ ٱلْتَقَى ٱلْجَمْعَانِ فَبِإِذْنِ ٱللَّهِ وَلِيَعْلَمَ ٱلْمُؤْمِنِينَ ﴾

“and all that befell you on the day when the two hosts met in battle happened by God's leave, so that He might mark out the [true] believers,”

Please check ayah 3:168 for complete tafsir.