You can find his fatawa on the muslim who joins the kuffar (Ibn Taymiyyah)
“Everyone who joins up with them – meaning with the Tartars – from the commanders of the armies and other than the commanders then his ruling is their ruling. There is found in them (the commanders) of apostasy from the legislations of Islām in proportion to what they (i.e. the Tartars) have apostated from. If the Salaf called those who withheld the Zakāh apostates, even though they would fast and pray and they did not fight against the Jamā’ah of the Muslims, what about the one who ended up with (joined) the enemies of Allāh and His Messenger fighting against the Muslims?” (Majmū’ al-Fatāwā (28/530).)
He also said:
“And He, the Most High, said in condemnation of the People of the Book:
تَرَىٰ كَثِيراً مِّنْهُمْ يَتَوَلَّوْنَ ٱلَّذِينَ كَفَرُواْ لَبِئْسَ مَا قَدَّمَتْ لَهُمْ أَنفُسُهُمْ أَن سَخِطَ ٱللَّهُ عَلَيْهِمْ وَفِي ٱلْعَذَابِ هُمْ خَالِدُونَ وَلَوْ كَانُوا يُؤْمِنُونَ بِالله والنَّبِيِّ وَمَا أُنْزِلَ إِلَيْهِ مَا اتَّخَذُوهُمْ أَوْلِيَآءَ وَلَـٰكِنَّ كَثِيراً مِّنْهُمْ فَاسِقُونَ
You see many of them taking the disbelievers as their allies. Evil indeed is that which their own selves have sent forward before them, for that (reason) Allāh’s Wrath fell upon them and in torment they will abide. And had they believed in Allāh, and in the Prophet and in what has been revealed to him, never would they have taken them (the disbelievers) as allies, but many of them are the criminals. [EMQ Sūrah al-Mā’idah (5):80-81.]
So He the Glorified and Most High, made it clear that Īmān in Allāh, the Prophet, and what He has sent down to him necessitates not taking them as protecting friends- so the establishment of their Wilāyah obligates an absence of Īmān because the absence of that which is necessitated dictates the absence of that which necessitates.” (Iqtidā as-Sirātil-Mustaqīm (1/221).)
And the nail in the coffin by Sheikh-ul Isllam Ibn taymiyyah
“And the likes of that, His words, the Most High, in another Āyah:
تَرَىٰ كَثِيراً مِّنْهُمْ يَتَوَلَّوْنَ ٱلَّذِينَ كَفَرُواْ لَبِئْسَ مَا قَدَّمَتْ لَهُمْ أَنفُسُهُمْ أَن سَخِطَ ٱللَّهُ عَلَيْهِمْ وَفِي ٱلْعَذَابِ هُمْ خَالِدُونَ وَلَوْ كَانُوا يُؤْمِنُونَ بِالله والنَّبِيِّ وَمَا أُنْزِلَ إِلَيْهِ مَا اتَّخَذُوهُمْ أَوْلِيَآءَ وَلَـٰكِنَّ كَثِيراً مِّنْهُمْ فَاسِقُونَ
You see many of them taking the disbelievers as their allies. Evil indeed is that which their own selves have sent forward before them, for that (reason) Allāh’s Wrath fell upon them and in torment they will abide. And had they believed in Allāh, and in the Prophet and in what has been revealed to him, never would they have taken them (the disbelievers) as allies, but many of them are the criminals. [EMQ Sūrah al-Mā’idah (5):80-81]
So mentioning a conditional sentence dictates that if the condition is found then the thing conditioned with the particle: (ْ لو) that implies along with the fulfillment of the condition the fulfillment of that which is conditioned. Hence He said:
وَلَوْ كَانُوا يُؤْمِنُونَ بِالله والنَّبِيِّ وَمَا أُنْزِلَ إِلَيْهِ مَا اتَّخَذُوهُمْ أَوْلِيَآءَ
And had they believed in Allāh, and in the Prophet and in what has been revealed to him, never would they have taken them (the disbelievers) as allies.
So that proves that the Īmān mentioned here negates and opposes taking them as allies and that Īmān and taking them as allies does not coincide in the heart. That proves that whoever takes them as allies has not implemented and acted upon the obligatory Īmān from Īmān in Allāh, His Prophet, and that which He has send down to him. This likes of it are His words, the Most High:
يَـٰأَيُّهَا ٱلَّذِينَ آمَنُواْ لاَ تَتَّخِذُواْ ٱلْيَهُودَ وَٱلنَّصَارَىٰ أَوْلِيَآءَ بَعْضُهُمْ أَوْلِيَآءُ بَعْضٍ وَمَن يَتَوَلَّهُمْ مِّنكُمْ فَإِنَّهُ مِنْهُمْ
O you who believe, do not take the Christians and Jews as allies, they are allies to one another and whoever is an ally to them from amongst you, then indeed he is from them.
For He has informed in those Āyāt that the one who allies himself with them is not a believer and He has informed here that the one who allies himself with them then he is from them hence, the Qur’ān confirms itself.”(Majmū’ al-Fatāwā (7/17))
I don't create these rulings, I simply follow
