ʿAlī b. Abī Tālib (d. 661 (AH 40)). Cousin and son-in-law of the Prophet Muḥammad, and the fourth Caliph (Khalīfa) in Islam. ʿAlī was one of the ten to whom paradise was promised by Muḥammad. He distinguished himself in all the early battles as a courageous warrior and was consequently nicknamed Haidar (lion) and Murtada (he in whom God is well pleased). During the rule of the first three Caliphs, ʿAlī served as an adviser. After ʿUthmān's murder, ʿAlī was proclaimed khalīfa (Caliph) by the Medinans. However, ʿAlī's reign was an unhappy and frustrating one, marked by the first civil war in Islamic history, the beginnings of the overt Sunni/Shīʿa split in Islam which persists to the present (Shīʿa being the party of ʿAlī).

Inconclusive military conflict at Siffīn (657 (AH 37)) led to the famous incident where the Syrians hoisted copies of the Qurʾān on spears, and invited the combatants to resolve the problem by recourse to the Holy Book. ʿAlī was forced to accept arbitration by most of his army, and was thus politically outmanœuvred by Muʿāwiyya's stratagem. ʿAlī's support diminished, a section of his army rebelled (those averse to arbitration) and these were crushed at Nahrawan (658 (AH 38)). The remnants of this defeated group later became known as the Kharijites. During ʿAlī's preparations for further battle, he was assassinated by a Kharijite in the mosque of Kūfa (661 (AH 40)).

Since the period of ʿAlī's caliphate is a controversial time in Islamic history, it is open to various interpretations. Both Sunni and Shīʿa sources agree that ʿAlī was a powerful orator, a leading authority on the Qurʾān and the Sunna of the Prophet Muḥammad, and that his piety was beyond question. ʿAlī's sermons, lectures, and discourses have been preserved in Nahj-ul-Balagha (collected in the 11th cent.). However, while Muḥammad, Abu Bakr, and ʿUmar had displayed great pragmatism in the handling of worldly affairs, ʿAlī lacked political insight: on the assumption of authority, he reversed all of ʿUthmān's policies, appointed new governors, initiated a programme of tribal reorganization, and moved the capital to Kūfa. ʿAlī's opponents constituted the Quraysh élite, particularly the Umayyah clan who had proved their administrative skills in the rapidly expanding empire. The Arab-Islam polity could only function with the co-operation of the Quraysh. The Muʿawiyya/ʿAlī struggle had deeper implications for the Islamic community than merely the avenging of ʿUthmān's murder, or the dominion of Syria or Iraq; it opened up the two different directions of the Muslim community. ʿAlī's programme was utopian, looking for the pure Islamic state, whereas that of Muʿawiyya was more of a secular nature.

In Shiʿite understanding, ʿAlī is walī Allah, ‘the friend of God’, closest to him in sanctity. As such he is distinguished from Muḥammad, who is (merely!), nabī, prophet. The family descent via ʿAlī designates the legitimate Imām, which can never, for a Shiʿite, be a matter of election—the most fundamental division from the Sunnis.