At-Tawbah 9:12

Allah says, if the idolators with whom you conducted peace treaties for an appointed term break

أَيْمَـنِهِمْ

(their oaths) meaning, terms of their treaties, and covenants

وَطَعَنُواْ فِى دِينِكُمْ

(and attack your religion...) with disapproval and criticism, it is because of this that one who curses the Messenger, peace be upon him, or attacks the religion of Islam by way of criticism and disapproval, they are to be fought. This is why Allah said afterwards,

فَقَـتِلُواْ أَئِمَّةَ الْكُفْرِ إِنَّهُمْ لاَ أَيْمَـنَ لَهُمْ لَعَلَّهُمْ يَنتَهُونَ

(then fight (you) against the leaders of disbelief -- for surely, their oaths are nothing to them -- so that they may stop.) so that they may refrain from the disbelief, rebellion and the transgression they indulge in. Qatadah and others said that the leaders of disbelief were Abu Jahl, `Utbah and Shaybah, Umayyah bin Khalaf, and he went on to mention several others. Al-A`mash narrated from Zayd bin Wahb from Hudhayfah; "The people of this Ayah were never fought again." A similar statement was reported from `Ali bin Abi Talib, may Allah be pleased with him. However, this Ayah is general, even though the specific reason behind revealing it was the idolators of Quraysh. So this Ayah generally applies to them and others as well, Allah knows best. Al-Walid bin Muslim said that Safwan bin `Amr narrated that `Abdur-Rahman bin Jubayr bin Nufayr said that when Abu Bakr sent an army to Ash-Sham, he advised them, "You will find some people with shaved heads. Therefore, strike the swords upon the parts that contain the devil, for by Allah, it is better to me to kill one of these people than to kill seventy other men. This is because Allah said,

فَقَـتِلُواْ أَئِمَّةَ الْكُفْرِ

(then fight (you) against the leaders of disbelief.)" Ibn Abi Hatim collected it.