Read and reflect on verse 587 from Surah An-Nisaa: [Hence,] O you who have attained to faith, when you go forth [to war] in God's cause, use your discernment, and do not - out of a desire for the fleet...
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا ضَرَبْتُمْ فِي سَبِيلِ اللَّهِ فَتَبَيَّنُوا وَلَا تَقُولُوا لِمَنْ أَلْقَىٰ إِلَيْكُمُ السَّلَامَ لَسْتَ مُؤْمِنًا تَبْتَغُونَ عَرَضَ الْحَيَاةِ الدُّنْيَا فَعِنْدَ اللَّهِ مَغَانِمُ كَثِيرَةٌ ۚ كَذَٰلِكَ كُنْتُمْ مِنْ قَبْلُ فَمَنَّ اللَّهُ عَلَيْكُمْ فَتَبَيَّنُوا ۚ إِنَّ اللَّهَ كَانَ بِمَا تَعْمَلُونَ خَبِيرًا
[Hence,] O you who have attained to faith, when you go forth [to war] in God's cause, use your discernment, and do not - out of a desire for the fleeting gains of this worldly life - say unto anyone who offers you the greeting of peace, "Thou art not a believer" for with God there are gains abundant. You, too, were once in the same condition - but God has been gracious unto you. Use, therefore, your discernment: verily, God is always aware of what you do.