Caliphates of Ali and Hasan, and the Umayyad era
During the First Fitna, Ibn Abbas remained a staunch supporter of his cousin, the Caliph Ali. Following the Battle of the Camel, Ali appointed him as the governor of Basra. He later fought prominently at the Battle of Siffin. After the fighting at Siffin ceased, Ali initially sought to appoint Ibn Abbas as his representative for the arbitration. However, this choice was rejected by Al-Ash'ath and the Iraqi qurra. They demanded a negotiator who was not a member of the Banu Hashim and who remained equally distant from both Ali and Mu'awiya . Following the assassination of Ali in 661 CE, Ibn Abbas pledged allegiance to his eldest son, Hasan ibn Ali, and remained a loyal supporter throughout Hasan's short caliphate, maintaining his allegiance until Hasan’s abdication in favor of Mu'awiya later that year.
During this period, a large segment of Ali's army broke away after becoming discontented with the arbitration, forming the faction known as the Kharijites. Ibn Abbas played a crucial role in engaging them, successfully convincing a large number of dissidents—numbering 20,000 out of 24,000 according to select sources—to return to Ali's fold by utilizing his deep knowledge of Muhammad's biography and scriptural interpretation. Documented across extensive medieval historiographical and biographical compendia such as al-Dhahabi's Siyar A'lam al-Nubala, Ibn Sa'd's Kitāb al-Ṭabaqāt al-Kabīr, Al-Mustadrak ala al-Sahihayn, and Musannaf Abd al-Razzaq, the theological rift centered around three core grievances raised by the Kharijites:
Al-Hakim al-Nishapuri in Al-Mustadrak ala al-Sahihayn, no. 2656,
Al-Nasa'i in As-Sunan al-Kubra no. 8522
Al-Nasa'i in Khoshoish Ali, no. 190
Al-Fasawi 1/522-524
al-Bayhaqi in Sunan al-Kubra (al-Bayhaqi), no. 16740
Musannaf Abd al-Razzaq, no. 18678
Al-Tabarani in Al-Mu'jam al-Kabir (Al-Tabarani), no. 10598
Diya' al-Din al-Maqdisi in Al-Āhādith al-Jiyād al-Mukhtārah min mā laysa fī Ṣaḥīḥain no. 436
Abu Nu'aym al-Isfahani in Hilyat al-Awliya', 3/318
Ali's policy for appointing Abu Musa al-Ash'ari as arbitrator in the dispute with Mu'awiyah. The Kharijites consider it as religious transgression, citing chapters Al-An'am Quran 6:57–7 (Translated by Pickthall) and Yusuf Quran 12:40–7 (Translated by Pickthall): "The decision rests with Allah only.". Ibn Abbas responded by pointing out chapter Al-Ma'idah Quran 5:95–7 (Translated by Pickthall). "the judge to be two men among you known for justice", arguing it is permitted to appoint arbitrator for dispute.
Ali's decision to not include his caliphal title during the arbitration with Mu'awiya's faction. Argued by Ibn Abbas, citing the Treaty of al-Hudaybiya when Muhammad does not include his honorific as Prophets and messengers in Islam when negotiating with Suhayl ibn Amr.
Spoils of war and captives, which was not taken by Ali after the Battle of the Camel, Ibn Abbas responded that since among the defeated was Aisha, the widow of Muhammad, it was impermissible for a Muslim to take another Muslim woman as a captive or to confiscate her property.
Following the death of Mu'awiya I in 680 CE (60 AH), Ibn Abbas turned his attention to political developments under the expanding Umayyad Caliphate. Before Husayn ibn Ali embarked on his expedition to Kufa, Ibn Abbas attempted to dissuade him, reminding him that the Kufans had previously abandoned both Ali and Hasan. He advised Husayn to seek refuge in Yemen instead, or at the very least, to leave his family behind if he insisted on going to Iraq.
