The initial divide between is immediately following the death of the prophet Muhammad. Fearing inserection and the splitting up of what Muhammad had united in Arabia two of his closest companions Abu Bakr and Umar travelled to Saqifah where a meeting between the leaders of the Ansar (Muslims from Medina who had asked Muhammad to govern their city) was taking place over who should succeed Muhammad. At that meeting Umar championed the cause of Abu Bakr who declared himself the first Khalifah (Caliph). This was done very quickly as power was already fracturing and led to the Ridda wars.

However 'Ali ibn Abi Talib, Muhammad's son in law through his marriage to Fatima, was not present at this meeting as he was attending to the affairs of Muhammad's funeral. Many people were unsatisfied that 'Ali's name hadn't been put forward at Saqifah and saw him as the natural successor to Muhammad as Muhammad did not leave any sons. The most well known endorsement of 'Ali is at Ghadir Khumm where whilst returning from his final pilgrimage Muhammad declared "Who I am the mawla of 'Ali is also his mawla" I've not translated the word "mawla" as its meaning is disputed here as Sunnis believe it to mean friend or close companion and Shias believe it to mean master or lord.

'Ali was unhappy that he had been overlooked for the role of Caliph but is generally believed to have accepted Abu Bakr as Caliph to maintain muslim unity. (Although there are some accounts of violence and/or threats against 'Ali and Fatima until he accepted the authority of Abu Bakr)

To skip over a few years Abu Bakr declared Umar to succeed him as the second Caliph. On his deathbed Umar conveined a group of 6 people including 'Ali and Uthman ibn Affan to elect the next Caliph within three days of his death.

Uthman won this election although votes were split entirely between Uthman and 'Ali, with 'Ali refusing to cast a vote.

During his time as Caliph Uthman put his trusted family and clan members in positions of power which upset a lot of the general population who saw it as nepotism. Eventually this boiled into civil unrest with groups petitioning for 'Ali to replace Uthman.

A group from Egypt travelled to Uthman to request he remove the governor of Egypt Abdullah ibn Sa'ad (Uthman's foster brother) for a variety of reasons. Uthman granted this but whilst journeying back to Egypt they noticed another man travelling behind them, after searching him they found a letter instructed Abdullah to remain governor and execute the men (this letter is widely considered a forgery from someone in Medina who was unsatisfied with Uthman's rule). The Egyptian delegation returned to question Uthman and when he denied writing the letter they laid siege to his house and he was eventually killed.

With Uthman dead many people petitioned 'Ali to become Caliph which he initially declined as some of his supporters were those who had taken arms against Uthman. He was eventually convinced to take the role by some of Muhammad's companions and the people of Medina.

After becoming Caliph he revoked the governorship of those appointed by Uthman. Notably this order was refused by Muawiyah.

In addition to displeasing those associated with Uthman (Umayyads) there were those who were upset that 'Ali had not punished those who killed Uthman including 'Aisha (one of Muhammad's wives) and Muawiyah. This led to a civil war known as the first Fitna. It's here where differences of opinion among Muslims begin to split into different sects.

Before the battle of the Camel in 656 CE 'Ali and 'Aisha met and appeared to reach a peaceful resolution, however during the night groups likely involved in the killing of Uthman, who at this point were present in both armies, started to kill people in both camps. Both believing they had been betrayed by the other, battle erupted the next day. 'Ali was victorious and 'Aisha was sent to Basrah, and later settled in Medina.

Muawiyah refused to acknowledge the authority of 'Ali over the levant and refused to accept 'Ali as Caliph as he had not been consulted regarding his rise. This led to the battle of Siffin in 657 CE. After three days of fighting both sides no longer wanted to fight each other as they were all Muslim. The sides agreed to arbitration however a group initially allied to 'Ali saw this as 'Ali failing as if God did support his cause he would be victorious without the need for arbitration. This group was called the Kharijites and eventually became the Ibadis (although modern Ibadis are keen to disassociate themselves from the movement). This group assassinated 'Ali in 661 CE.

'Ali's son al-Hasan was elected Caliph after the death of his father and after about 6 months he abdicated to end the split between those that supported Muawiyah and those that supported 'Ali. This was on the condition that al-Hasan succeed Muawiyah and Muawiyah not name an heir. It is possible that al-Hasan was poisoned by his wife on Muawiyah's orders as al-Hasan 20 years younger and very likely to outlive him. After al-Hasan's death Muawiyah named his son Yazid as his heir on the basis that al-Hasan couldn't succeed him and there needed to be an obvious successor.

During Yazid's time as Caliph another of 'Ali's sons Hussein felt that Yazid was an unjust and unfit ruler. In Hussein's view he was honourbound to fight against oppression even against insurmountable odds. He gathered some loyal companions and they were massacred at Karbala in 680 CE. After this event the Shia formed into a distinct and arguably irreconcilable group in their support for Hussein and his descendants.