In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Allaah [The Exalted] said:

فَلۡيُقَـٰتِلۡ فِى سَبِيلِ ٱللَّهِ ٱلَّذِينَ يَشۡرُونَ ٱلۡحَيَوٰةَ ٱلدُّنۡيَا بِٱلۡأَخِرَةِ‌ۚ وَمَن يُقَـٰتِلۡ فِى سَبِيلِ ٱللَّهِ فَيُقۡتَلۡ أَوۡ يَغۡلِبۡ فَسَوۡفَ نُؤۡتِيهِ أَجۡرًا عَظِيمً۬ا
وَمَا لَكُمۡ لَا تُقَـٰتِلُونَ فِى سَبِيلِ ٱللَّهِ وَٱلۡمُسۡتَضۡعَفِينَ مِنَ ٱلرِّجَالِ وَٱلنِّسَآءِ وَٱلۡوِلۡدَٲنِ ٱلَّذِينَ يَقُولُونَ رَبَّنَآ أَخۡرِجۡنَا مِنۡ هَـٰذِهِ ٱلۡقَرۡيَةِ ٱلظَّالِمِ أَهۡلُهَا وَٱجۡعَل لَّنَا مِن لَّدُنكَ وَلِيًّ۬ا وَٱجۡعَل لَّنَا مِن لَّدُنكَ نَصِيرًا
ٱلَّذِينَ ءَامَنُواْ يُقَـٰتِلُونَ فِى سَبِيلِ ٱللَّهِ‌ۖ وَٱلَّذِينَ كَفَرُواْ يُقَـٰتِلُونَ فِى سَبِيلِ ٱلطَّـٰغُوتِ فَقَـٰتِلُوٓاْ أَوۡلِيَآءَ ٱلشَّيۡطَـٰنِ‌ۖ إِنَّ كَيۡدَ ٱلشَّيۡطَـٰنِ كَانَ ضَعِيفًا

Let those (believers) who sell the life of this world for the Hereafter fight in the Cause of Allah, and whoso fights in the Cause of Allah, and is killed or gets victory, We shall bestow on him a great reward. And what is wrong with you that you fight not in the Cause of Allah, and for those weak, ill-treated and oppressed among men, women, and children, whose cry is: “Our Lord! Rescue us from this town whose people are oppressors; and raise for us from You one who will protect, and raise for us from You one who will help.” Those who believe, fight in the Cause of Allah, and those who disbelieve, fight in the cause of Taghut. So fight you against the friends of Shaitan (Satan); Ever feeble indeed is the plot of Shaitan (Satan). [Surah An-Nisaa. Aayat 74-76]

[وَٱلَّذِينَ كَفَرُواْ يُقَـٰتِلُونَ فِى سَبِيلِ ٱلطَّـٰغُوتِ – Those who disbelieve fight in the cause of Taghut]- Taghut here means shaytaan. (1) Those who wilfully deny the Oneness of Allaah, belie Allaah’s Messenger and that which He has brought from his Lord, they fight in the path of Taghut – meaning, they obey shaytaan, follow his way and methodology which he has legislated for his friends amongst the proponents of disbelief. (2) [إِنَّ كَيۡدَ ٱلشَّيۡطَـٰنِ كَانَ ضَعِيفًا – Ever feeble indeed is the plot of Shaitan]- Meaning, when he forsook his companions on the day of badr. (3) Allaah stated about the Day of Badr:

وَإِذۡ زَيَّنَ لَهُمُ ٱلشَّيۡطَـٰنُ أَعۡمَـٰلَهُمۡ وَقَالَ لَا غَالِبَ لَڪُمُ ٱلۡيَوۡمَ مِنَ ٱلنَّاسِ وَإِنِّى جَارٌ۬ لَّڪُمۡ‌ۖ فَلَمَّا تَرَآءَتِ ٱلۡفِئَتَانِ نَكَصَ عَلَىٰ عَقِبَيۡهِ وَقَالَ إِنِّى بَرِىٓءٌ۬ مِّنڪُمۡ إِنِّىٓ أَرَىٰ مَا لَا تَرَوۡنَ إِنِّىٓ أَخَافُ ٱللَّهَ‌ۚ وَٱللَّهُ شَدِيدُ ٱلۡعِقَابِ

And (remember) when Shaitan (Satan) made their (evil) deeds seem fair to them and said, “No one of mankind can overcome you this Day (of the battle of Badr) and verily, I am your neighbour (for each and every help).” But when the two forces came in sight of each other, he ran away and said “Verily, I have nothing to do with you. Verily! I see what you see not. Verily! I fear Allah for Allah is Severe in punishment.” [Surah Al-Anfaal. Aayah 48]

Allaah described them to be weak because they neither fight out of hope for Allaah’s reward nor do they fear punishment when they abandon fighting; rather they fight solely due to fanatism or harbouring evny against the believers due to that which Allaah has bestowed on them of His favours (Islam, Iman, the religion of truth). (4)

Read Jihaad in our era- By Shaikh Abu Khadeejah from the clarifications of Al-Allaamah Saaleh Al-Fawzaan: https://abukhadeejah.com/salafi-shaikh-fawzaan-on-jihaad-in-our-times-and-the-guidelines-of-jihaad-according-to-islam/

[Ref 1: Zaadul Maseer Fee Ilmit Tafseer]
[Ref 2: Tafseer at-Tabari]
[Ref 3: Zaadul Maseer Fee Ilmit Tafseer]
[Tafseer at-Tabari]