The scholars of Islam have stated that it is not permitted to support the kaafirs against the Muslims, and that that is kufr (disbelief) and riddah (apostasy), because Allaah says (interpretation of the meaning):
“O you who believe! Take not the Jews and the Christians as Awliyaa’ (friends, protectors, helpers), they are but Awliyaa’ of each other. And if any amongst you takes them (as Awliyaa’), then surely, he is one of them”
The fuqaha’ of Islam, the imams of the Hanafi, Maaliki, Shaafa’i and Hanbali madhhabs, and other fuqaha’, have stated in their books that it is haraam to sell them anything that may help them against the Muslims, such as weapons, equipment, riding beasts, etc. So it is not permissible to give them food or to sell them food, drink, water, tents, trucks, vehicles, or to make contracts with them to provide maintenance, transaportation, etc. All of that is haraam and the one who consumes profits on such transactions is consuming haraam things, and the Fire is more fitting for him.
It is not permissible to sell them even a date or to give them anything that they can use against their enemies. Whoever does that deserves Hell, and the Fire is more fitting for all evil earnings. Indeed this is one of the most evil of evil things.
It is not permissible to give them anything that may give them the slightest help against the Muslims.
Al-Nawawi said in al-Majmoo’:
With regard to selling weapons to ahl al-harb (those who wage war against the Muslims), it is haraam according to scholarly consensus.
Ibn al-Qayyim said in I’laam al-Muwaqqi’een:
Imam Ahmad said: The Messenger of Allaah (peace and blessings of Allaah be upon him) forbade selling weapons at times of fitnah (tribulation)… It is obvious that selling them is helping others in sin and transgression. This also applies to every sale, rental or exchange that helps people to disobey Allaah, such as selling weapons to kaafirs, aggressors and bandits… or renting one’s house to someone who will set up a place for sin in it, or selling candles to someone who will use them to disobey Allaah, and other actions which help people to do that which Allaah hates and is angry with.
In al-Mawsoo’ah al-Fiqhiyyah (25/153) it says:
It is haraam to sell weapons to ahl al-harb (those who are waging war against Islam) or to those who are known as bandits who attack Muslims or who stir up fitnah among them. Al-Hasan al-Basri said: It is not permissible for a Muslim to take weapons or horses to the enemies of the Muslims which will strengthen them against the Muslims, or anything that will help them to acquire weapons and horses, because selling weapons to ahl al-harb strengthens them to fight the Muslims and motivates them to declare war and continue fighting.
This issue is not the matter of ordinary or minor sins, rather it is a matter that has to do with the basis of ‘aqeedah (belief) and Tawheed (belief in the Oneness of Allaah), and the Muslim’s support and loyalty towards the Religion of Allaah and his disavowal of the enemies of Allaah. This is what was stated by the imams in their books.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said in his Fataawa (1/274):
The scholars of Islam are unanimously agreed that whoever supports the kaafirs against the Muslims and helps them in any way is a kaafir like them as Allaah says (interpretation of the meaning):
“O you who believe! Take not the Jews and the Christians as Awliyaa’ (friends, protectors, helpers), they are but Awliyaa’ of each other. And if any amongst you takes them (as Awliyaa’), then surely, he is one of them. Verily, Allaah guides not those people who are the Zaalimoon (polytheists and wrongdoers and unjust)”