Ibn Taymiyyah said:
“It has been established on the authority of the Quran , the Sunnah, and consensus, that those who depart from the law of Islam must be fought, even if they pronounce the two professions of faith.
The Fuqaha (Jurists) disagree about the impressibility to fight rebellious groups that abandon an established supererogatory act of worship (Sunnah Raatibah), such as the two (extra) rak’ahs of Fajr prayer. There is, however, unanimity that it is allowed to fight people for (not observing) unambiguous and generally recognized obligations and prohibitions, until they undertake to perform the explicitly prescribed prayers, to pay zakah, to fast the month of Ramadhan, to make the pilgrimage to makkah and to avoid what is prohibited, such as marrying women in spite of legal impediments, eating impure things, acting unlawfully against the lives and properties of Muslims and the like.
It is obligatory to take the initiative in fighting those people, as soon as the Prophet’s summons with the reasons for which they are fought has reached them. But if they attack the Muslims, then fighting them is even more urgent…”
[Taken from, As-Siyaasah Ash-Shariyyah Fee Islaah Ar-Raa’ee Wal-Ra’iyyah”, by Ibn Taymiyyah]