The political reasons behind the so-called religious infighting.
Muslims are divided along many lines, with the Sunni-Shi’a split being the most prominent. The split has existed for over a thousand years and is motivated primarily by political interests, rather than submission to any divine command.
The split began immediately after the death of Mohammed. Shi’a Muslims argue Mohammed appointed Ali ibn Abi-Talib to be his successor, and Ali was denied his rightful position due to the conspiracy that occurred at Saqifah. They also propose, because God chose Mohammed, Mohammed has the right to select his successor due to his supposed divine authority. Sunni Muslims deny this and claim, Mohammed was supportive of the community (in this sense, Mohammed’s political circle,) deciding his successor through debate. Following Mohammed’s death, Abu Bakr became the next leader. Both Ali and Abu were senior members within Mohammed’s inner circle. They were part of the ruling class. The Sunni-Shi’a divide, therefore, was first and foremost a fight between the political elite to secure leadership and power.
Following Mohammed’s death, leadership changed hands multiple times within a short period. Recognition or rejection of the authority of these leaders is a key aspect of the Sunni-Shi’a divide. Abu Bakr was the 1st Caliph. Abu was the father of Mohammed’s wife, Aisha. Due to the conspiracy he partook in at Saqifah and not being selected for leadership by Mohammed, Shi’a Muslims do not recognise Abu Bakr’s authority. Two years into his leadership, Abu Bakr was assassinated, and leadership was assumed by Umar ibn al-Khattab – father of Mohammed’s wife, Hafsa. Umar was also assassinated after 10 years and the leadership passed to Uthman ibn ‘Affan – husband of Mohammed daughter Ruqayya and later Umm Kulthum. Uthman ruled for 12 years and then he was assassinated. Ali ibn Abi-Talib became the 4th Caliph. By Shi’a account, Ali was selected by Mohammed. He was also Mohammed’s first cousin and husband to Mohammed daughter Fatimah. Both Sunni and Shi’a Muslims recognise his authority, but Shi’a Muslims consider him the 1st Caliph, whereas Sunni Muslims consider him to be the 4th. Highlighting the divide between Sunni and Shi’a sects of Islam. To Sunnis, the de facto power holder is the legitimate leader; to Shi’a Muslims only de jure leaders are legitimate.
Ali was assassinated, and leadership was taken by his son, Hussein ibn Ali. Hussein ruled for several months then abdicated following an agreement with Muawiyah ibn Abi Sufyan: Muawiyah agreed to appoint him the next Caliph and make annual contributions from the Treasury. Muawiyah, however, violet these terms: Hussein did not receive money from the Treasury, and he appointed his son Yazid ibn Muawiya as successor.
Following the latter violation, Hussein leads a rebellion against Muawiyah’s successor, Yazid. At the battle of Karbala Hussein was defeated and killed.
At this time supporters of Ali and later Hussein adopted the narrative that Caliph should remain with the family of the prophet. This is not something Mohammed advocated. It is purely a product of power struggles between societal elites.
Two centuries into the Umayyad dynasty, a man named Abu al-Abbas claims descent from Mohammed, raises an army and defeats the Umayyads. Those who believe Abbas’s claim of descent and recognise his authority are known as Sunni Muslims. Those who don’t are Shi’a Muslims.
In the initial phase, one sect of the then political elite argued leadership should stay with Ali as Mohammed chose him. Half of a century later the narrative changed to, ‘leadership should remain with Mohammed’s lineage’. As infighting continues, political elites adopt positions which furthers their own interest. And in this power struggle, just as in modern politics, self-serving ideas are passed off as normative truths.