After Allah (swt) mentioned that He would cast terror into the hearts of the infidels, He now begins to talk about the Companions (may Allah be pleased with them), who lived this incident:

وَلَقَدْ صَدَقَكُمُ اللّهُ وَعْدَهُ إِذْ تَحُسُّونَهُم بِإِذْنِه {And Allah had certainly fulfilled His promise to you when you were killing the enemy by His permission}: There was a victory at the beginning of the battle because as we know that the archers left their positions to take the spoils and considered that the war had ended, so at the beginning of Allah (swt) fulfilled His promise and gave them victory because Allah (swt) says: {when you were killing the enemy}.

حَتَّى إِذَا فَشِلْتُمْ وَتَنَازَعْتُمْ فِي الأَمْرِ وَعَصَيْتُم مِّن بَعْدِ مَا أَرَاكُم مَّا تُحِبُّونَ {until [the time] when you lost courage and fell to disputing about the order [given by the Prophet] and disobeyed after He had shown you that which you love}: After you have seen the victory.

مِنكُم مَّن يُرِيدُ الدُّنْيَا{Among you are some who desire this world}: Those who disobeyed the commands of the Prophet (saws), and went to the booty.

وَمِنكُم مَّن يُرِيدُ الآخِرَةَ{and among you are some who desire the Hereafter.}: So there was a promise of victory, and they were victorious, but when they violated the Prophet (saws), they were defeated. Abdullah bin Masoud (May Allah be pleased with him) said: If I swore on that day, I would hope that my oath was true that none of us wanted the world, until then Allah sent down: {Among you are some who desire this world, and among you are some who desire the Hereafter.}.

وَلَقَدْ عَفَا عَنكُمْ {And He has already forgiven you,}: Look at the kindness and the mercy of Allah (swt), these are the believers who have weakened and committed disobedience, but they did not abandon the divine commands, so Allah (swt) pardoned them.