The eldest son of Ali and Fatima and a grandson of the Islamic prophet Muhammad, Hasan briefly ruled as Rashidun caliph from January 661 until August 661.

Given Mu'awiya's military superiority, supporters of Hasan maintain that his abdication was inevitable after his soldiers mutinied and that he was motivated by the desire for unity and peace among Muslims, which was reportedly predicted by Muhammad in a Sunni hadith.

[4] Their union holds a special spiritual significance for Muslims, write Nasr and Afsaruddin,[5] and Muhammad said he followed divine orders to marry Fatima to Ali, narrates the Sunni al-Suyuti (d. 1505), among others.

"[14] After an inconclusive debate in 10/631-2, Muhammad and the Najranite Christians decided to engage in mubuhala, where both parties would pray to invoke God's curse upon the liar.

[32] Shias hold that she miscarried her child and died from the injuries she suffered in an attack on her house,[6][33][34] intended to subdue Ali,[35] at the order of Abu Bakr.

[49] Haj-Manouchehri writes that Hasan persuaded some neutral figures to support Ali at Siffin, including Sulayman ibn Surad al-Khuza'i.

[51][10] Madelung writes that Ali had apparently not nominated a successor before his sudden death but had often said that only members of Muhammad's household (ahl al-bayt) were entitled to the caliphate.

[53] Some authors have noted that Muhammad's surviving companions were primarily in Ali's army and must have therefore pledged allegiance to Hasan, as evidenced by the lack of any reports to the contrary.

[64][63] Mu'awiya replied that he was better suited for the caliphate because of his age, governing experience, and superior military strength,[65][63][66] thus implying that these qualities were more important than religious precedence.

As the news of Mu'awiya's advance reached Hasan, he ordered his local governors to mobilize and invited the Kufans to prepare for war, "God had prescribed the jihad for his creation and called it a loathsome duty (kurh)," referring to verse 2:216 of the Quran.

[2] This is the view of al-Zuhri (d. 741-2),[72] the Umayyad-era historian who adopted the pro-Umayyad account that depicts a greedy Hasan eager to renounce his caliphate for money.

[73] This must have been the official Umayyad account, distributed to legitimize Mu'awiya's rule in the absence of a council (shura) or election or designation (nass), suggests Jafri.

Among the five surviving accounts, Jafri prefers the one by Abu Hanifa Dinawari (d. 895), which states that Hasan was concerned about his troops' resolve by the time he reached the outskirts of al-Mada'in.

"[10][76][78] Al-Jarrah was overpowered and killed,[78] while Hasan, bleeding profusely,[10] was taken for treatment to the house of Sa'd ibn Mas'ud al-Thaqafi, the governor of al-Mada'in.

[85][10] When Hasan learned about this, al-Ya'qubi writes that he summoned the Iraqi nobles and reproached them for their unreliability and fickle-mindedness, echoing the speeches of Ali after Siffin.

She lists several conditions in the early sources and questions their veracity, including an annual payment of one or two million dirhams to Hasan, a single payment of five million dirhams from the treasury of Kufa, annual revenues from variously named districts in Persia, succession of Hasan to Mu'awiya or a council (shura) after Mu'awiya, and preference for the Banu Hashim over the Banu Umayyad in pensions.

[2] As reported by the Mu'tazilite Ibn Abi'l-Hadid (d. 1258) and Abu al-Faraj al-Isfahani (d. 967), Mu'awiya added that he had not fought the Iraqis so that they would practice Islam, which they were already doing, but to be their master (amir).

[116] These reports are nevertheless suppressed by al-Tabari, perhaps because he found them insignificant or far more likely because he was concerned for the faith of the common people (awamm) in this and similar instances, as suggested by Madelung and Donaldson (d. 1976).

Jafri suggests that the age difference presented a problem for Mu'awiya,[114] who planned to designate his son Yazid (r. 680–683) as his successor,[121][114] in violation of the peace treaty with Hasan.

[10] The Shia al-Mufid (d. 1022) reports that Hasan's wife Ja'da bint al-Ash'ath poisoned him with the promise of 100,000 dirhams from Mu'awiya and marriage to his son Yazid.

Using mineralogical, medical, and chemical evidence, they suggested that the mineral calomel (mercury(I) chloride, Hg2Cl2), sourced from the Byzantine Empire, was the substance primarily responsible for Hasan's death.

The authors thus conclude that their forensic hypothesis is consistent with the historical narrative that Hasan was poisoned by his wife Ja'da at the instigation of Mu'awiya and with the involvement of the Byzantine emperor.

[2] As Hasan's body was carried to al-Baqi, however, Marwan reportedly joined the procession and paid tribute to a man "whose forbearance (hilm) weighed mountains.

[63] Other sources reject these criticisms, saying that Hasan's abdication was inevitable after the Kufans' mutiny, similar to Ali's acceptance of the arbitration proposal at Siffin (657).

[159][161] Shia historians view Hasan's abdication as the only realistic course of action, given the Kufans' weak support and Mu'awiya's overwhelming military superiority.

The Mu'tazilite al-Qadi Abd al-Jabbar (d. 1025) suggests that Hasan reluctantly made peace and unwillingly pledged his allegiance to Mu'awiya after realizing the Kufans' weak support for war.

The Mu'tazilite Ibn al-Malahimi (d. 1141) adds, "How can it be imagined that Hasan, who planned to fight Mu'awiya to secure his oath of allegiance, would agree to relinquish the caliphate without reluctance?

[190] Another hypothesis may have included Hasan as the fifth rāshid caliph because his six-month reign was needed to complete the thirty-year period after Abu Bakr's ascension which was reportedly predicted by Muhammad as the length of the prophetic succession.

Poets include Sanai (Hadiqat al Haqiqa), Attar of Nishapur, Ghavami Razi, Rumi, 'Ala' al-Dawla Simnani, Ibn Yamin, Khwaju Kermani, Salman Savoji, Hazin Lahiji, Naziri Neyshabouri, Vesal Shirazi, and Adib al-Malak Farahani.

[203] The series Loneliest Leader, directed in 1996 by Mehdi Fakhimzadeh, narrates Hasan's life, his peace with Mu'awiya, and the condition of the Islamic community after his assassination.