Previous answer.

Possibly you can advise what you are having trouble with ? For an /r/AskHistorians expert to elaborate, if necessary ?

The usual response to this question is something along the lines of the “choice between who should succeed or Muhammad upon his death, or if he should be succeeded at all”. With the Sunnis generally saying Abu Bakr and Shias saying Ali. However, I think this is hugely reductive and misses the context given during the life of Muhammad. It also fails to address the nuance of what beliefs were acceptable to be put in either group after his death.

The Tl;dr

Sunnis accept the first three Caliphs after Muhammad, whereas Shias reject them. This leads to the different groups having different perceptions about who’s authority is legitimate on how the religion should be practiced. They also accept and reject different narrators as reliable for transmitting history and doctrine and so have come to slightly different methods of practice and acceptable beliefs.

My answer proper:

During the life of Muhammad:

The first thing we should look to is where is the final point that both groups agree on what is the correct interpretation of Islam. Which is of course during the life of Muhammad. So can either group claim any proofs on Muhammad’s intentions on who should succeed him?

The Sunnis can appeal to a couple of arguments for the intended succession of Abu Bakr. Mostly from the account of Muhammad’s wife and Abu Bakr’s daughter Aisha. Firstly she reported “The Messenger of God told me during his illness: Call your father Abu Bakr and your brother to me so I might write a letter. For I fear that someone will have wishful fancies and someone will say: I am more worthy, but God and the faithful refuse anyone but Abu Bakr.”

There are various narrations from Aisha along these lines stating Muhammad’s preference for Abu Bakr to succeed him, another notable one involving Muhammad instructing Abu Bakr to lead a prayer (the implication being that should Muhammad be unable to in future then Abu Bakr should).

The Shia can also appeal to various events, notably Ghadir Khum, were Muhammad declared “Do I not have more authority over the believer than they have over themselves?” To which the gathered crowd returning from the Hajj replied in the affirmative. Then going on to say “Whoever takes me as their mawla, take Ali as their mawla.” Mawla I have left untranslated as it has multiple possible translations. The Shia believe it to mean master or leader whilst Sunnis say it means friend.

Immediately upon Muhammad’s death:

After the death of Muhammad a group of Ansar (Medinans who had converted to Islam) gathered in Saqifah Bani Saida to discuss what should happen with regard political and religious authority now that the prophet had passed. Abu Bakr and Umar joined by some of the Muhajirun (those that followed Muhammad’s migration from Mecca to Medina) went to Saqifah where Umar put forward Abu Bakr’s claim and secured the obedience of those present (excepting some who refused to follow anyone but Ali). There are various contradictory telling of what exactly happened and how but generally this is understood to be the bare bones of what happened. Notably Ali was not present at this meeting as he was preparing Muhammad’s body for burial.

Upon discovering that Abu Bakr had taken the title of Khalifah (successor, Caliph) several supporters of Ali’s claim gathered at his and Fatima’s (Muhammad’s daughter and Ali’s wife) house. There are reports that Umar went to the house with armed men to secure Ali’s allegiance to Abu Bakr. The violence in this episode varies widely depending on who narrates it with some tellings saying Ali gave his allegiance immediately, and others that Umar threatened to burn the house down and struck Fatima, causing her to miscarry. Based on most narrations it’s likely that Ali did not give his allegiance to Abu Bakr until six months later, following the death of Fatima (whom he buried in secret, giving credence to the idea there was an issue between them and Umar); and even when he did give his allegiance, he refused to do so with Umar present.

For brevity I will skip the talking about the reigns of Abu Bakr and Umar save that several decisions were made that Shias perceive as insults to Ali and that Sunnis interpret as legitimate enforcement of shariah. As Umar, who had become khalifah after Abu Bakr, was dying after being stabbed by Abu Lulu. He appointed a council of five members on who should succeed him. Abd al-Rahman bin Awf, Sad bin Abi Waqqas, Zubayr ibn al-Awwam, Uthman and Ali. A sixth, Talha was also chosen for this council but was unable to attend. This council found in favour of Uthman’s claim, with Ali reluctantly agreeing.

Uthman, compared to his predecessors was relatively weak, favouring his own tribe (the Umayyads) and family in governance. A group of rebels rose against him, laid siege to his house and ultimately killed him. Ali and his sons Hasan and Hussein had attempted to prevent the rebels gaining access to Uthman. But when Uthman was killed and all three of Ali’s household survived some Quarayshis questioned the genuineness of Ali’s attempt to protect the Caliph.

The reign of Ali Battle of the camel:

After Uthman’s murder the two obvious choices for the next khalifah were Talha, and Ali. Initially it appeared as if Talha had been declared khalifah; however, after a short time Talha would give his allegiance to Ali.

Ali’s early reign was difficult; with Uthman’s killers melting back into the population and other factions demanding justice for Uthman. Ali did not rush to punish the killers as there was much confusion as to who they were and he feared meting out unjust punishments. Aisha, who had left Mecca in the belief that Talha had succeeded to the Caliphate, immediately returned upon hearing of Ali’s ascension, and accused him of murdering Uthman. Upon hearing this many prominent Quarayshis broke their allegiance to Ali, including Talha.

Ali would make further enemies by stripping governors appointed by Uthman of their titles, notably Muawiya; Uthman’s cousin and a prominent Umayyad.

Talha, Zubayr and Aisha went on to lead an army in rebellion against Ali in the name of avenging Uthman. Some three and half thousand men took arms against Ali for this cause. When the rebel army came to Basra they pleaded their case for Uthman’s justice and the town was split, some in support for the rebels, others calling them liars. The next morning the Basran governor led an attack on the rebels which ultimately failed and the two sides agreed to wait for Ali’s arrival before taking further action.

The rebels would break this agreement, seizing, imprisoning, and humiliating the governor as he led the evening prayer. The rebels went on to take full control of the town, however the population generally maintained their preference for Ali, refusing to take up arms against him.

Ali would rally forces to his side on his way to Basra. When he arrived a tent was pitched between the two armies who faced each other for three days before the battle began. Ali ordered a messenger to hold a copy of the Qur’an between the battlelines, pleading for concord. When the messenger was killed by arrows, Ali ordered the attack. Zubayr fled the battle seemingly without participating but was later killed during his flight, Talha was killed by a man named Marwan from within his own forces. Marwan’s reasoning being that Talha was in part responsible for Uthman’s death.

With the two rebel leaders killed the battle had all but been decided with the rebel forces now much more focused on defending Aisha, who was seated on a camel, than anything else. The fighting continued until the camel was hamstrung by Ali’s forces. He sent Aisha’s brother Muhammad to retrieve her. He greatly admonished her before forgiving her and sending her back to her home.

(Continued in reply)