in Islāmic usage, a heretical uprising, especially the first major internal struggle within the Muslim community (AD 656–661), which resulted in both civil war and religious schism—between the Sunnites (Sunnite) and Shīʿites (Shīʿite).

The third caliph, ʿUthmān (Uthmān ibn ʿAffānʿ) (reigned 644–656), a member of the Umayyad family of Mecca, had incurred the opposition of Muḥammad's closest followers, the Muslims of Medina, by favouring his own Meccan family in his official appointments. ʿUthmān's murder by Egyptian soldiers (June 17, 656) elicited Meccan demands for revenge, and when Muḥammad's son-in-law, Alīʿ, whom the Medinese had proclaimed fourth caliph, failed to comply, opposition was directed against him. The Battle of the Camel (December 656), pitting the forces of ʿAlī against those of Āʾishahʿ, one of Muḥammad's widows, and Ṭalḥah and az-Zubayr, prominent Companions of the Prophet, temporarily secured ʿAlī's position but inaugurated civil war. Muʿāwiyah (Muʿāwiyah I), another Umayyad from Mecca and governor of Syria, took up the demands for vengeance on ʿUthmān's death and questioned the validity of ʿAlī's caliphate. Their confrontation in the Battle of Ṣiffīn (Ṣiffīn, Battle of) (657), which the arbitration at Adhruḥ (659) attempted to resolve, was disastrous: it split ʿAlī's forces, some of his followers (Khawārij (Khārijite)) refusing to acknowledge the validity of human arbitration in a case which they felt could be rightly decided only by God. ʿAlī's position was also undermined when the arbitrators would not declare him the rightful caliph; the result was an irrevocable split in Islām by the formation of the shīʿat ʿAlī (“party of ʿAlī”), political allies of ʿAlī who eventually translated their political demands into a religious conviction that ʿAlī and all his descendants were divinely appointed to succeed Muḥammad as caliphs. Strengthened by this outcome, Muʿāwiyah seized Egypt and began raiding ʿAlī's stronghold, Iraq. The open warfare finally ended in 661 when ʿAlī was assassinated and Muʿāwiyah began his reign as the first Umayyad caliph, but the religious split endured between the Sunnites and the Shīʿites.