In which century did the Sunni and Shi'a branches of Islam formalize their split?

The formalization of the split between the Sunni and Shi'a branches of Islam primarily occurred in the seventh century, shortly after the death of the Prophet Muhammad in 632 CE. This division stemmed from a disagreement over the rightful successor to Muhammad. The majority of Muslims, who would become known as Sunnis, believed that the leadership should be based on consensus and the community's choice, leading to the selection of Abu Bakr, a close companion of Muhammad, as the first caliph.

In contrast, the Shi'a faction believed that leadership should remain within the Prophet's family, specifically favoring Ali, the Prophet's cousin and son-in-law. This theological and political dispute laid the groundwork for a significant rift in the early Islamic community, which has persisted through the centuries. The formalization of these positions and the sociopolitical ramifications of this division marked a foundational moment in Islamic history during the seventh century. Subsequent developments, including battles and political intrigues, further solidified the differences between the two branches, leading to the rich and complex historical and cultural landscapes evident in today’s Muslim world.