In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Shaikhul Islaam Ibn Taymiyyah [rahimahullaah] said: And because of this [i.e. the destructive consequences of fighting during fitna], the Prophet [sallal-laahu-alayhi-wasallam] did not praise anyone who participated in fighting at the battle of Al-Jamal and Siffeen, let alone what took place in Madeenah on the day of Al-Harrah; neither [is there any praise regarding] the siege against Ibn Zubayr nor what took place in the fitna of Ibn Al-Ash’ath, Ibn Al-Muhallab and other than them; rather it has been reported from the Prophet [sallal laahu alayhi wasallam] though a sizable number of narrators that the khawaarij are to be fought – those whom Ali [Ameerul Mumineen] fought against at Nahrawaan after they rebelled against him at Harooraa.
The narrations in the Sunnah reported from the Prophet [sallal-laahu-alayhi-wasallam] shows in detail that they [i.e. Khawaarij] are to be fought against. Therefore, Ali [radiyallaahu-anhu] was happy when he fought them, and he narrated ahaadeeth about them. All the Sahaabah agreed to fight them and likewise the Imaams of the people of knowledge after them. They did not consider this fighting [against the Khawaarij] a fitna, as opposed to the fighting at Al-Jamal, As-Siffeen and other incidents of fighting which has no text to justify them- neither through the consensus of the scholars [of Ahlus Sunnah] nor were those incidents of fighting praised by those virtuous people who participated in them, rather they were regretful [afterwards] and abandoned it.
Likewise, Al Hasan always used to indicate to his father and brother [Al Husayn] to abandon fighting. And when the affair was left in his [jurisdiction], he abandoned fighting and brought about peace between the two fighting parties. Likewise, it became clear to Ali [radiyallaahu-anhu] in the latter part of the affair that to abandon fighting carried more benefit than fighting. Also, Al-Husayn [radiyallaahu-anhu] was not murdered except due to being oppressed and killed as a martyr, after he abandoned seeking after authority and sought to return to his country, or to the seaport, or to be taken to Yazeed the ruler. Listen to audio- (only fifteen minutes)- about how the shia betrayed al Husayn (radiyallaahu-anhu): https://safeshare.tv/x/MdsrmMzdTj Also visit: Everything You Need to Know About The History and Doctrine of The Rafidi Shia Backstabbers And Exposition of Rafidi State Propaganda Against the People of Tawheed and Sunnah http://www.shia.bs/index.cfm
Source: An Excerpt from Silsilah Ar-Rasaa’il Al-Manhajiyyah Fin-Nus’hi Wal-Irshaad Wat-taw’iyah’ pages 30-33 slightly paraphrased