The Spoils of War
ذَٰلِكَ بِأَنَّهُمْ شَآقُّوا۟ ٱللَّهَ وَرَسُولَهُۥ ۚ وَمَن يُشَاقِقِ ٱللَّهَ وَرَسُولَهُۥ فَإِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ
That is because they opposed Allah and His Messenger. And whoever opposes Allah and His Messenger - indeed, Allah is severe in penalty.
Nearby ayahs help place verse 8:13 in sequence and meaning.
إِذْ يُغَشِّيكُمُ ٱلنُّعَاسَ أَمَنَةً مِّنْهُ وَيُنَزِّلُ عَلَيْكُم مِّنَ ٱلسَّمَآءِ مَآءً لِّيُطَهِّرَكُم بِهِۦ وَيُذْهِبَ عَنكُمْ رِجْزَ ٱلشَّيْطَـٰنِ وَلِيَرْبِطَ عَلَىٰ قُلُوبِكُمْ وَيُثَبِّتَ بِهِ ٱلْأَقْدَامَ
[Remember] when He overwhelmed you with drowsiness [giving] security from Him and sent down upon you from the sky, rain by which to purify you and remove from you the evil [suggestions] of Satan and to make steadfast your hearts and plant firmly thereby your feet.
إِذْ يُوحِى رَبُّكَ إِلَى ٱلْمَلَـٰٓئِكَةِ أَنِّى مَعَكُمْ فَثَبِّتُوا۟ ٱلَّذِينَ ءَامَنُوا۟ ۚ سَأُلْقِى فِى قُلُوبِ ٱلَّذِينَ كَفَرُوا۟ ٱلرُّعْبَ فَٱضْرِبُوا۟ فَوْقَ ٱلْأَعْنَاقِ وَٱضْرِبُوا۟ مِنْهُمْ كُلَّ بَنَانٍ
[Remember] when your Lord inspired to the angels, "I am with you, so strengthen those who have believed. I will cast terror into the hearts of those who disbelieved, so strike [them] upon the necks and strike from them every fingertip."
ذَٰلِكُمْ فَذُوقُوهُ وَأَنَّ لِلْكَـٰفِرِينَ عَذَابَ ٱلنَّارِ
"That [is yours], so taste it." And indeed for the disbelievers is the punishment of the Fire.
يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا لَقِيتُمُ ٱلَّذِينَ كَفَرُوا۟ زَحْفًا فَلَا تُوَلُّوهُمُ ٱلْأَدْبَارَ
O you who have believed, when you meet those who disbelieve advancing [for battle], do not turn to them your backs [in flight].