Shaykh Ibn ʿUthaymīn (رحمه الله) said:
It is obligatory upon the subjects to support the imām (ruler) in fighting them (the rebels), and even more so, the Khārijites. It is obligatory upon the subjects to support the imām in fighting them because they desire to split the ummah. Even if they were to say, “We want rectification”, then they are (in reality) corruptors on earth.
01 The ruler fighting against the Khārijites and rebels is from the well-known foundations of Ahl al-Sunnah and the books of creed are replete with the mention of fighting alongside the ruler, under his authority. When there is a clear Muslim authority, and there is a clear armed rebellion, then it is a collective obligation to support the ruler against the rebels.
02 The scholars define rebels (bughāt) as an armed group with power and strength which rebels against the ruler, refraining from obedience that is obligatory upon them, on account of a plausible (but faulty) interpretation. It is necessary to first listen to their grievances and their doubts, for reconciliation, so the issues can be addressed and they can be answered and can be advised to come under the ruler’s obedience. If they refuse after that, then they are to be fought, the foundation for the being the verse (49:9-).
03 The difference between the Khārijites and the rebels (bughāt) is that the latter exit from the rulers obedience due to a plausible interpretation (taʾwīl sāʾigh) in which they lack the evidence, the evidence is not on their side. Whereas the Khārijites who rebel have no plausible interpretation, since they rebel on account of a deviant belief (takfīr of Muslims), in which they declare the rulers to be disbelievers and believe it is necessary to remove them. So they rebel and fight based on a doctrine, whereas the bughāt rebel and fight based on an interpretation in which the evidence is not on their side.[1]
04 Given that Khārijites and rebels, if left alone, would kill and take Muslims captive, as well as violate sanctities, then it is obligatory upon the ruler to fight against them after attempts to rectify them. And Shaykh Ibn ʿUthaymīn explained that the subjects are under a collective obligation to support the authorities in that, as a means of putting a swift end to the corruption.
05 This is from the clear affairs whose ruling the scholars of the Muslims have explained and it is not from the fighting of “fitnah”, wherein Muslims should refrain and sit at home. The fighting of “fitnah” is a different category and includes fighting against oppressive rulers or fighting when the side of truth is not known, or fighting when greater evil will result and so on.
06 It is the duty of the people of knowledge, religiosity and reason in the land to support the ruler and to explain this obligation to the people, to the army and to the ruler’s subjects, that this is from the legislated fighting and is jihād in Allāh’s path.