Shaykh Saalih al-Fawzaan said, “There are a number of categories of assisting the disbelievers against the Muslims;
1) (The allegiance in which there is Tawalle is the following), to help them and assist them (the kuffaar) against the Muslims (i.e. help them, aid them or give them victory over the Muslims), while loving what they are upon of disbelief (kufr) (loving their religion or ideologies), polytheism (shirk), and misguidance. This category, without doubt is major disbelief that causes one to exit the religion.
2) (The allegiance in which there is Muwalaat is the following) When one (a Muslim) helps the disbelievers against the Muslims by his own free will, without being forced, while he still hates the religion of the disbelievers and is not pleased with it. Such a person, no doubt, has committed one of the major sins, and we fear that he may fall into (major) disbelief. Had he not hated them and their religion we (the scholars of Ahlus Sunnah) would have ruled upon him with kufr (disbelief). Thus, he is in great danger.
3) When one loves the disbelievers even without helping them against the Muslims. Allah has prohibited this (and in fact it is major kufr. But if it is natural love such; as a Muslim son loving his non-Muslim parents or a Muslim man who marries a Christian woman, he has natural love for her, then Allah has not prohibited this and it is not major kufr).
4) When one helps the disbelievers against other disbelievers who have a contract of safety with the Muslims (through the UN for example). This is not permissible since it causes the contract of the Muslims to be broken.
So it is not permissible for any of the Muslims to fight the disbelievers who are under such a contract, so as to honour the contract that is between them and the Muslims.
Thus, the one who assists some disbelievers in fighting against them (the disbelievers who have a contract of safety with the Muslims) has caused the Muslims contract to become breached and he has betrayed the honour of the Muslims.”
Ref: Taken from his book Duroos fee Shara Nawaaqidh al-Islaam
(MY WORDS – Abdul Kareem Ibn Ozzie – This is article by Shaykh Al Fawzaan is not an in depth article but a summarised look at majority of the categories of aiding the kuffar against Muslims.
Hence the Shaykh has not discussed Muslims helping the kuffar against Muslims due to being forced or Muslims helping/seeking help from the kuffar against transgressive Muslim groups (who seek to or are engaged in killing other innocent Muslims without justification).)
Question to Shaykh Al-Fawzaan: Is it always allowed for one group of Muslims to ally with another group of Muslims against the kuffaar who there is a contract of safety with? (Example: USA attack Iraq so in this situation can Saudi Arabia ally with the Iraq’s against the USA even though Saudi and the USA have contract of safety with each other).
Answer: Shaykh Saalih al-Fawzaan said, “…if some Muslims seek our (other Muslims) assistance against some disbelievers then we must assist the Muslims against the disbelievers, expect in one situation, when those disbelievers have a contract of safety with the Muslims. In this case it is not permissible for us (the Muslims who were sort for assistance) to assist the Muslims against them, so then how could we assist the disbelievers against those who have contracts of safety with the Muslims? This is something that is not permissible, due to (the obligation of) upholding contracts.”
Taken from his book Duroos fee Shara Nawaaqidh al-Islaam (p.56-159)
Posted by Abdul Kareem Ibn Ozzie