Hassan ibn Ali was a prominent early Islamic figure. He was the eldest son of Ali and Fatima and the grandson of the Islamic prophet Muhammad. He briefly reigned as caliph from January 661 to August 661. He is considered the second Imam in Shiite Islam, succeeding Ali and preceding his brother Hussein. As the grandson of the prophet, he is part of the ahl al-bayt and ahl al-kisa, and is also said to have participated in the Mubahal event.
During Ali's caliphate (656–661), Hasan accompanied him in the military campaigns of the First Muslim Civil War. After the assassination of Ali in 661, Hasan was recognized as caliph in Kufa. His sovereignty was not recognized by the governor of Syria, Muawiyah I (661–680), who led an army into Kufa, urging Hasan's abdication in letters.
In response, Hasan sent a vanguard commanded by Ubayd Allah ibn al-Abbas to block Mu'awiyah's advance until he arrived with the main army. Meanwhile, Hasan was severely injured in a failed assassination attempt by the Kharijites, a faction opposed to Ali and Mu'awiyah. This attack demoralized Hasan's army and led to mass desertion. Ubayd Allah and most of his troops also deserted after Muawiya bribed him.
In August 661, Hasan signed a peace treaty with Mu'awiyah on the condition that the latter rule in accordance with the Qur'an and Sunnah, the council appoint his successor, and Hasan's supporters receive an amnesty. Hassan retired from politics and abdicated in Medina, where he died either from illness or poisoning, although early sources are almost unanimous that he was poisoned. Mu'awiya is usually considered the instigator of the murder of Hasan, which removed the obstacle in the way of his son Yazid (680–683).
Hasan's critics call his treaty with Mu'awiya a sign of weakness, saying he intended to surrender from the start. Given Mu'awiyah's military superiority, Hasan's supporters argue that his abdication was inevitable after his soldiers rebelled, and that he was driven by a desire for unity and peace among Muslims, which was reportedly foretold by Muhammad in a Sunni hadith. Another Sunni hadith, also attributed to Muhammad, predicted that the prophetic succession would last thirty years, which was probably interpreted by some early Sunni scholars as evidence that Hassan's caliphate was the right one (rashid).
In Shia theology, Hassan's divine infallibility (isma) as the second Shia Imam further justified his course of action. As Muhammad's rightful successor in Shia Islam, Hassan's overarching temporal and religious authority came from an inspired appointment (nass) that was not annulled by the abdication in favor of Mu'awiya, who usurped only temporal power. The Imamate and Caliphate are treated as separate institutions in Shia Islam until God makes the Imam victorious.
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