The Battle of Siffin (Arabic: معركة صفّين, romanized: Maʿraka Ṣiffīn) was fought in 657 CE (37 AH) between the Iraqi Arab forces of the fourth Rashidun caliph Ali ibn Abi Talib and the Syrian Arab forces of the rebelling long-time governor of the Levant, Mu'awiya ibn Abi Sufyan. The battle is named after its location Siffin on the banks of the Euphrates river. The fighting stopped after the Syrians called for arbitration when Ali's forces started to gain the upper hand, to which Ali reluctantly agreed under pressure from some of his troops. The arbitration process ended inconclusively in 658 though it strengthened the Syrians' support for Mu'awiya and his political position while weakening the authority of Ali and causing dissension among the Iraqis. The battle was part of the First Fitna and is considered a major step towards the establishment of the Umayyad Caliphate.

The battlefield was located in Siffin, a ruined Byzantine-era village at the right bank of the Euphrates in the vicinity of Raqqa in present-day Syria. It has been identified with the modern village of Tell Abu Hureyra in Raqqa Governorate.

Ali frequently accused the third Rashidun caliph Uthman of deviating from the Quran and the Sunna, and was joined in his criticism by many senior companions of the Islamic prophet Muhammad, including Talha and Zubayr. Uthman was also widely accused of nepotism, corruption, and injustice, and Ali is known to have protested his conduct, including his lavish gifts for his kinsmen, the Umayyads. Ali also protected outspoken companions, such as Abu Dharr al-Ghifari and Ammar ibn Yasir, from Uthman. Ali appears in early sources as a restraining influence on Uthman without directly opposing him. Some supporters of Ali were also part of the opposition to Uthman, joined in their efforts by Talha and Zubayr, as well as Aisha, a widow of Muhammad. Aisha was critical of Uthman for religious innovations and nepotism, while also objecting his reduction of her pension. Among the supporters of Ali were Malik al-Ashtar (d. 657) and other religiously-learned qurra (lit. 'Quran readers'). These wanted to see Ali as the next caliph, though there is no evidence that he communicated or coordinated with them. Ali is also said to have rejected the requests to lead the rebels, although he might have sympathized with their grievances, and was thus considered a natural focus for the opposition, at least morally. It is also likely that some companions supported the protests with the hope of either deposing Uthman, or changing his policies, thus underestimating the severity of the opposition to Uthman.

As their grievances mounted, discontented groups from provinces began arriving in Medina in 35/656. On their first attempt, the Egyptian opposition sought the advice of Ali, who urged them to send a delegation to negotiate with Uthman, unlike Talha and Ammar, who might have encouraged the Egyptians to advance on the town. Ali similarly asked the Iraqi opposition to avoid violence, which was heeded. He also acted as a mediator between Uthman and the provincial dissidents more than once to address their economic and political grievances. In particular, he negotiated and guaranteed on behalf of Uthman the promises that persuaded the rebels to return home and ended the first siege. Ali then urged Uthman to publicly repent, which he did. The caliph soon retracted his statement, however, possibly because his secretary Marwan convinced him that repentance would only embolden the opposition. On their way back home, some Egyptian rebels intercepted an official letter ordering their punishment. They now returned to Medina and laid siege to Uthman's residence for a second time, demanding that he abdicate. The caliph refused and claimed he was unaware of the letter, for which Marwan is often blamed in the early sources. Ali and another companion, Muhammad ibn Maslama, sided with Uthman about the letter, and suspected Marwan, while a report by the Sunni historian al-Baladhuri (d. 892) suggests that the caliph accused Ali of forging the letter. This is likely when Ali refused to further intercede for Uthman. That Ali was behind the letter is also the opinion of the Islamicist Leone Caetani (d. 1935). Among other Western historians, Giorgio Levi della Vida (d. 1967) is unsure, while Wilferd Madelung strongly rejects the accusation, saying that it "stretches the imagination" in the absence of any evidence. In turn, he accuses Marwan, the bellicose secretary of Uthman, while Hugh N. Kennedy holds Uthman responsible for the letter. The caliph was assassinated soon afterward in the final days of 35 AH (June 656) by the Egyptian rebels, during a raid on his residence in Medina.

Ali played no role in the deadly attack, and his son Hasan was injured while guarding the besieged residence of Uthman at the request of Ali. He also convinced the rebels not to prevent the delivery of water to Uthman's house during the siege. Beyond this, historians disagree about his measures to protect the third caliph. Ali is represented by the Sunni historian al-Tabari (d. 923) as an honest negotiator genuinely concerned for Uthman. Madelung and the modern historian Husain M. Jafri (d. 2019) highlight the attempts by Ali for reconciliation, and the Islamicist Martin Hinds (d. 1988) believes that Ali could not have done anything more for Uthman. The Islamic author Reza Shah-Kazemi points to Ali's "constructive criticism" of Uthman and his opposition to violence, while the Bahai scholar Moojan Momen writes that Ali mediated between Uthman and the rebels, urging the former to alter his policies and refusing the requests from the latter to lead them. This is similar to the view of the Islamicist John McHugo, who adds that Ali withdrew in frustration when his peace efforts were thwarted by Marwan. The historians Fred Donner and Robert Gleave suggest that Ali was the immediate beneficiary of Uthman's death. In turn, their opinion is challenged by Madelung, who argues that Aisha would have not actively opposed Uthman if Ali had been the prime mover of the rebellion and its future beneficiary. He and others observe the hostility of Aisha toward Ali, which resurfaced immediately after his accession in the Battle of the Camel (656). The orientalist Laura Veccia Vaglieri (d. 1989) believes that Ali refused to lead the rebellion but still sympathized with them and possibly agreed with their calls for Uthman's abdication. Among other scholars, Hossein Nasr and Asma Afsaruddin, Levi della Vida, and Julius Wellhausen (d. 1918) believe that Ali remained neutral in this conflict, while Caetani labels Ali as the chief culprit in the murder of Uthman, even though the evidence suggests otherwise. The Muslim scholar Mahmoud M. Ayoub (d. 2021) notes the often pro-Umayyad stance of the Western classical orientalists, with the exception of Madelung.

When Uthman was killed in 656 CE by the Egyptian rebels, the potential candidates for the caliphate were Ali and Talha. The Umayyads had fled Medina, and the provincial rebels and the Ansar (early Medinan Muslims) were in control of the city. Among the Egyptians, Talha enjoyed some support, but the Basrans and Kufans, who had heeded Ali's call for nonviolence, and most of the Ansar supported Ali. Some authors add the (majority of the) Muhajirun (early Meccan Muslims) to this list of Ali's supporters. The key tribal chiefs also seem to have favored Ali at the time. The caliphate was thus offered by these groups to Ali, who was initially reluctant to accept it, saying that he preferred to be a minister (wazir). Soon after, possibly after it became clear that he enjoyed popular support, Ali did accept the caliphate, demanding a public pledge at the mosque. One factor in his decision to accept the caliphate might have been to prevent further chaos, but his nomination by the rebels nevertheless left Ali exposed to accusations of complicity in the assassination of Uthman. It also appears that Ali personally did not force anyone for pledge and, among others, Sa'd ibn Abi Waqqas, Abd Allah ibn Umar, Sa'id ibn al-As, Al-Walid ibn Uqba, and Marwan likely refused to give their oaths, some possibly motivated by their personal grudges against Ali. On the whole, Madelung suggests that there is less evidence for any violence here than in the case of the first caliph, Abu Bakr (r. 632–634), even though many broke with Ali later, claiming that they had pledged under duress. At the same time, that the rebels favored Ali might have created an intimidating atmosphere in Medina for those opposed to him.

At the time of the assassination, the key governorships were in the hands of the Umayyads, and their often-late conversion to Islam might have suggested expediency to Ali and the Ansar. Ali was nevertheless advised to initially confirm the governors of Uthman, some of whom were unpopular, to consolidate his authority over the caliphate. He rejected this advice and replaced nearly all those who had served Uthman, saying that the likes of those men should not be appointed to any office. In this and other decisions, Ali was driven by his sense of religious mission, writes Madelung, while Poonawala suggests that Ali changed the governors to please the rebels. Donner has a similar view to Madelung and Shah-Kazemi maintains that justice was the key principle that molded Ali's policies in all domains. Even so, Madelung views this decision of Ali as politically naive. His view is in turn rejected by the Shia scholar Ali Bahramian, who believes that replacing the governors was the only available course of action, both on principle and in practice. He contends that injustice was the main grievance of the provincial rebels and they would have turned against Ali had he confirmed Uthman's governors. Among these governors was Uthman's cousin Mu'awiya, who had been appointed as a local governor in the Levant by the second caliph Umar (r. 634–644), and then risen to become the sole governor of Greater Syria under Uthman. Having ruled in Syria for almost twenty years, without interruption and almost since its conquest, Mu'awiya was firmly entrenched in Syria with a strong power base which made his removal difficult. Ali replaced Mu'awiya with his cousin Abd Allah ibn Abbas, who feared the retaliation of Mu'awiya and asked Ali to postpone his appointment until his rule was firmly established, according to al-Baladhuri.

There are some reports of early correspondence, and al-Baladhuri writes about a letter by Ali that asked Mu'awiya to report to him in Medina. Alternatively, Madelung suggests that Ali contacted Mu'awiya only after arriving in Kufa, following his victory in the Battle of the Camel. He might have waited this long perhaps to have the upper hand after his victory or perhaps because Mu'awiya was not a serious contender for the caliphate anyway, considering that he was a taliq (those pardoned by Muhammad when Mecca fell to Muslims), the son of Abu Sufyan, who had led the confederates against Muslims, and Hind, who was responsible for mutilating the body of Muhammad's uncle Hamza. For his part, Mu'awiya had waited through the Battle of the Camel, and was perhaps emboldened by that challenge to the authority of Ali.