Hasan ibn Ali (Arabic: , romanized: asan ibn Al; c.625 2 April 670) was the eldest son of Ali and Fatima and a grandson of the Islamic prophet Muhammad. He briefly ruled as caliph from January 661 until August 661. He is considered the second Shia Imam, succeeding Ali and preceding his brother, Husayn. In Sunni Islam, Hasan is considered one of the greatest sahaba and part of Muhammad's family. Part of the ahl al-bayt and the ahl al-kisa, he participated in the event of Mubahala. Muhammad described Hasan and Husayn as "the masters of youth in Paradise".

During the caliphate of Ali (r. 656661), Hasan accompanied him in the military campaigns of the First Muslim Civil War. After Ali's assassination in 661, Hasan was subsequently acknowledged caliph in Kufa. His sovereignty was unrecognized by Syria's governor Mu'awiya I (r. 661680), who led an army into Kufa to press the former for abdication. In response, Hasan sent a vanguard under Ubayd Allah ibn al-Abbas to block Mu'awiya's advance until he arrived with the main army. Meanwhile, Hasan was severely wounded in an abortive assassination attempt made by the Kharijites, a faction opposed to both Ali and Mu'awiya. This attack demoralized Hasan's army, and many deserted. On the other hand, Ubayd Allah and most of his troops defected after Mu'awiya bribed him. In August 661 (Rabi al-Thani 41 AH), Hasan signed a peace treaty with Mu'awiya on the basis that the latter should rule in compliance with the Quran and the sunna, a council appointed his successor, and his supporters received amnesty. Hasan retired from politics and abdicated in Medina where he eventually died either by a long-term illness or poisoning at the instigation of Mu'awiya, who wanted to install his son Yazid (r. 680683), as his successor.

Critics of Hasan call his treaty with Mu'awiya an indication of weakness, saying that he had intended to surrender from the beginning and fought half-heartedly. His supporters maintain that Hasan's abdication was inevitable after his soldiers mutinied, and he was motivated by the desire for unity and peace in the Muslim community; Muhammad reportedly predicted that Hasan would make peace among the Muslims. Another Sunni hadith (also attributed to Muhammad) predicted that the prophetic succession would endure for thirty years, which may have been interpreted by at least some early Sunni scholars as evidence that Hasan's caliphate was rightly-guided (rshid). In Shia theology, the divine infallibility (isma) of Hasan as the second Shia Imam justified his course of action. Shiites do not consider Hasan's resignation from political power detrimental to his imamate, which is based on Nass (divine designation by Muhammad and Ali). In Shia theology, the imamate is not transmissible to another person by allegiance or voluntary resignation.



Shīʿa Islam or Shīʿīsm is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated ʿAlī ibn Abī Ṭālib as his successor (khalīfa) and the Imam (spiritual and political leader) after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions (ṣaḥāba) at Saqifah. This view primarily contrasts with that of Sunnī Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abū Bakr, who was appointed caliph by a group of senior Muslims at Saqifah, to be the first rightful (rāshidūn) caliph after Muhammad. Adherents of Shīʿa Islam are called Shīʿa Muslims, Shīʿītes, or simply Shīʿa or Shia.Shīʿa Islam is based on a ḥadīth report concerning Muhammad's pronouncement at Ghadir Khumm. Shīʿa Muslims believe that ʿAlī ibn Abī Ṭālib, Muhammad's cousin and son-in-law, should have been the designated successor to Muhammad as Islam's spiritual and political leader. This belief later developed into the concept of Imamah, the idea that certain descendants of Muhammad, the Ahl al-Bayt, are rightful rulers or Imams, whom Shīʿa Muslims believe possess special spiritual and political authority over the Muslim community. Although there are many Shīʿa subsects, modern Shīʿa Islam has been divided into two main groupings: Twelvers and Ismāʿīlīs, with Twelver Shīʿas being the largest and most influential group among Shīʿa Muslims.Shīʿa Islam is the second largest branch of Islam, followed by 10–15% of all Muslims. Twelver Shīʿīsm is the largest branch of Shīʿa Islam, comprising about 85% of all Shīʿa Muslims.