Ḥasan ibn ʻAlī, -approximately 669

Hasan ibn Ali (Arabic: حسن ابن علي, romanized: Ḥasan ibn ʿAlī; 4 March 625 – 5 March 670 CE) 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. Hasan is considered as the second Imam in Shia Islam, succeeding Ali and preceding his brother Husayn. As a grandson of the prophet, he is part of the ahl al-bayt and the ahl al-kisa, and also participated in the event of the mubahala. Muhammad described him and his brother, Husayn, as the leaders of the youth of paradise. During the caliphate of Ali (r. 656–661), Hasan accompanied him in the military campaigns of the First Fitna. Following Ali's assassination in January 661, Hasan succeeded him in Kufa. His sovereignty was not recognized by Mu'awiya I (r. 661–680), a rival claimant to the caliphate, who invaded Iraq with his Syrian army while pressing Hasan to abdicate. Hasan sent a vanguard under Ubayd Allah ibn Abbas to block Mu'awiya's advance until he arrived with the main army. Meanwhile, Hasan was wounded in an abortive assassination attempt by a member of the Kharijites, a faction opposed to both Ali and Mu'awiya. The attack demoralized Hasan's army and led to widespread desertion, including by Ubayd Allah and most of his troops. In August 661, Hasan signed a peace treaty, ceding the caliphate to Mu'awiya on the condition that the latter would rule in compliance with the Quran and the Sunnah, and that a council would appoint his successor. Hasan withdrew from politics and retired to Medina, where he died either from illness or poisoning. In the early sources, which are nearly unanimous that he was poisoned, Mu'awiya is viewed as the instigator, to remove an obstacle to the succession of his son Yazid I (r. 680–683). Sunni scholars consider Hasan to have been motivated by the desire for unity and peace among Muslims, which was reportedly predicted by Muhammad in a hadith. Another Sunni hadith, also attributed to Muhammad, predicted that the prophetic succession would last for thirty years, which has been interpreted by some Sunni scholars as evidence that Hasan's caliphate was rightly-guided. In Shia theology, Hasan's infallibility (isma) as the second Imam justified his course of action. His all-inclusive temporal and religious authority came from divine designation (nass), which was not annulled by abdication to Mu'awiya, who usurped only the temporal authority.