Al-'amr bi al-ma'ruf wa al-nahy 'an al-munkar is an essential Islamic duty, unanimously accepted by all Muslims. The difference occurs only in the limits and conditions related to it.
For instance, the Khawarij believed in it without any limits and conditions whatsoever. They believed that this twofold duty must be performed in all circumstances. For example, when others believed in the conditions of probability of effectiveness (of al-ma'ruf) and absence of any dangerous consequences as necessary for this obligation to be applicable, the Khawarij did not believe in any such restrictions. Some believed that it is sufficient to fulfil the duty of al-'amr wa al-nahy by the heart and the tongue i e one should support al-ma'ruf and oppose al-munkar in his heart and use his tongue to speak out for al-ma'ruf and against al-munkar. But the Khawarij considered it incumbent to take up arms and to unsheathe one's sword for the sake of fulfilling this duty.
As against them there was a group which considered al-'amr wa al-nahy to be subject to the above conditions, and, moreover, did not go beyond the confines of the heart and the tongue for its sake. Ahmad ibn Hanbal is counted among them. According to this group,a bloody uprising for the sake of struggling against unlawful activities is not permissible.
The Mu'tazilah accepted the conditions for al-'amr wa al-nahy, but, not limiting it to the heart and the tongue, maintained that if the unlawful practices become common, or if the state is oppressive and unjust, it is obligatory for Muslims to rise in armed revolt.
Thus the belief special to the Mu'tazilah in regard to al-'amr bi al-ma'ruf wa al-nahy 'an al-munkar - contrary to the stand of the Ahl al-Hadith and the Ahl al-Sunnah - is belief in the necessity to rise up in arms to confront corruption. The Khawarij too shared this view, with the difference pointed out above.
source : http://www.muslimphilosophy.com/ip/kalam.htm
