Husayn ibn Ali in the context of "Shi'a"

Shia Islam is the second-largest branch of Islam. It holds that Muhammad designated Ali ibn Abi Talib (r. 656–661) as both his political successor (caliph) and as the spiritual leader of the Muslim community (imam). However, his right is understood to have been usurped by a number of Muhammad's companions at the meeting of Saqifa, during which they appointed Abu Bakr (r. 632–634) as caliph instead. As such, Sunni Muslims believe Abu Bakr, Umar (r. 634–644), Uthman (r. 644–656) and Ali to be 'rightly-guided caliphs', whereas Shia Muslims regard only Ali as the legitimate successor.

Shia Muslims believe that the imamate continued through Ali's sons, Hasan and Husayn, after which various Shia branches developed and recognized different imams. They revere the ahl al-bayt, the family of Muhammad, maintaining that they possess divine knowledge. Shia holy sites include the shrine of Ali in Najaf, the shrine of Husayn in Karbala, and other mausoleums of the ahl al-bayt. Later events, such as Husayn's martyrdom in the Battle of Karbala (680 CE), further influenced the development of Shia Islam, contributing to the formation of a distinct religious sect with its own rituals and shared collective memory.

The Second Fitna was the second civil war in the Islamic community during the early Umayyad Caliphate. It followed the death of the first Umayyad caliph Mu'awiya I in 680, and lasted for about twelve years. The war involved the defeat of three main challenges to the authority of the Umayyad dynasty, the first by Husayn ibn Ali, as well as his supporters including Sulayman ibn Surad and Mukhtar al-Thaqafi who rallied to avenge his death in Iraq, the second by Abd Allah ibn al-Zubayr who proclaimed himself caliph in Mecca and Medina and would be nominally recognized throughout most of the Caliphate, and the Kharijites who took over central Arabia and southern Iraq and Persia.

The roots of the civil war go back to the First Fitna. After the assassination of the third Rashidun caliph Uthman, the Islamic community experienced its first civil war over the immediate question of retribution for his murder. Following the assassination of the fourth Rashidun caliph Ali in 661 and the abdication of his successor Hasan the same year, Mu'awiya became the sole ruler of the caliphate. Mu'awiya's unprecedented decision to nominate his son Yazid as his heir sparked opposition, and tensions soared after Mu'awiya's death. Husayn ibn Ali was invited by the pro-Alids of Kufa to overthrow the Umayyads but was killed with his small company en route to Kufa at the Battle of Karbala in October 680. Yazid's army suppressed anti-government rebels in Medina in August 683 and subsequently besieged Mecca, where Ibn al-Zubayr had established himself in opposition to Yazid. After Yazid died in November the siege was abandoned, and Umayyad authority would soon collapse throughout the caliphate following the death of his son except in parts of Syria where Marwan I was proclaimed caliph; most provinces recognized Ibn al-Zubayr as caliph. A series of pro-Alid movements demanding to avenge Husayn's death emerged in Kufa, beginning with Ibn Surad's Penitents movement, which was crushed by the Umayyads at the Battle of Ayn al-Warda in January 685. Kufa was then taken over by Mukhtar, who rallied Husayn's supporters and the disenfranchised mawali to his cause. Though his forces routed a large Umayyad army at the Battle of Khazir in August 686, Mukhtar and his supporters were slain by the Zubayrids in April 687 following a series of battles. Under the leadership of Marwan, the Umayyads would consolidate their power over Syria and retake Egypt from the Zubayrids, and under his successor Abd al-Malik ibn Marwan, the Umayyads would reconquer Iraq after defeating the Zubayrids at the Battle of Maskin in 691 and would reassert their control over the Caliphate after killing Ibn al-Zubayr in the second siege of Mecca in 692, while their general Al-Hajjaj ibn Yusuf would suppress the Kharijites in the years afterwards.

Marwan ibn al-Hakam ibn Abi al-As ibn Umayya (Arabic: مروان بن الحكم بن أبي العاص بن أمية, romanized: Marwān ibn al-Ḥakam ibn Abī al-ʿĀṣ ibn Umayya; 623 or 626 – April/May 685), commonly known as Marwan I, was the fourth Umayyad caliph, ruling for less than a year in 684–685. He founded the Marwanid ruling house of the Umayyad dynasty, which replaced the Sufyanid house after its collapse in the Second Fitna and remained in power until 750.

During the reign of his cousin, the third Rashidun caliph Uthman (r. 644–656), Marwan took part in a military campaign against the Byzantine Exarchate of Africa (in central North Africa), where he acquired significant war spoils. He also served as Uthman's governor in Fars (southwestern Iran) before becoming the caliph's katib (secretary or scribe). He was wounded fighting the rebel siege of Uthman's house, in which the caliph was slain. In the ensuing civil war between the fourth Rashidun caliph Ali (r. 656–661) and the largely Qurayshite partisans of Aisha, Marwan sided with the latter at the Battle of the Camel. Marwan later served as governor of Medina under his distant kinsman Caliph Mu'awiya I (r. 661–680), founder of the Umayyad Caliphate. During the reign of Mu'awiya's son and successor Yazid I (r. 680–683), Marwan organized the defense of the Umayyad realm in the Hejaz (western Arabia) against the local opposition which included prominent companions as well as Muhammad’s own clan, the Bani Hashim, who revolted under the banner of Muhammad’s grandson, Husayn ibn Ali. After Yazid died in November 683, the Mecca-based rebel and sahabi Abd Allah ibn al-Zubayr declared himself caliph and expelled Marwan, who took refuge in Syria, the center of Umayyad rule. With the death of the last Sufyanid caliph Mu'awiya II in 684, Marwan, encouraged by the ex-governor of Iraq Ubayd Allah ibn Ziyad, volunteered his candidacy for the caliphate during a summit of pro-Umayyad tribes in Jabiya. The tribal nobility, led by Ibn Bahdal of the Banu Kalb, elected Marwan and together they defeated the pro-Zubayrid Qays tribes at the Battle of Marj Rahit in August of that year.

Fatima bint Muhammad (Arabic: فَاطِمَة بِنْت مُحَمَّد, romanized: Fāṭima bint Muḥammad; 605/15–632 CE), commonly known as Fatima al-Zahra' (Arabic: فَاطِمَة ٱلزَّهْرَاء, romanized: Fāṭima al-Zahrāʾ), was the daughter of the Islamic prophet Muhammad and his wife Khadija. Fatima's husband was Ali, the fourth of the Rashidun caliphs and the first Shia imam. Fatima's sons were Hasan and Husayn, the second and third Shia imams, respectively. Fatima has been compared to Mary, mother of Jesus, especially in Shia Islam. Muhammad is said to have regarded her as the best of women and the dearest person to him. She is often viewed as an ultimate archetype for Muslim women and an example of compassion, generosity, and enduring suffering. It is through Fatima that Muhammad's family line has survived to this date. Her name and her epithets remain popular choices for Muslim girls.

When Muhammad died in 632, Fatima and her husband Ali refused to acknowledge the authority of the first caliph, Abu Bakr. The couple and their supporters held that Ali was the rightful successor of Muhammad, possibly referring to his announcement at the Ghadir Khumm. Controversy surrounds Fatima's death within six months of Muhammad's. Sunni Islam holds that Fatima died from grief. In Shia Islam, however, Fatima's miscarriage and death are said to have been the direct result of her injuries during a raid on her house to subdue Ali, ordered by Abu Bakr. It is believed that Fatima's dying wish was that the caliph should not attend her funeral. She was buried secretly at night and her exact burial place remains uncertain.

Alids (Arabic: العلويين, romanized: al-ʿAlawiyyīn) are those who claim descent from Ali, the fourth Rashidun caliph (r. 656–661), the first imam in Shia Islam. Ali was also the cousin and son-in-law of the Islamic prophet Muhammad.

The main branches are the Hasanids and Husaynids, named after Hasan and Husayn, the eldest sons of Ali from his marriage to Fatima, the daughter of Muhammad. As the progeny of Muhammad, they are revered by all Muslims. The Alids have led various movements in Islam, and a line of twelve Alids are the imams in Twelver Shia, the largest Shia branch.