What was the main issue that led to the split between Shi'a and Sunni Muslims in the seventh century?

The split between Shi'a and Sunni Muslims in the seventh century was primarily rooted in a disagreement over how the rightful leaders of the Muslim community should be chosen. This division emerged soon after the death of the Prophet Muhammad in 632 CE. The central issue revolved around the legitimacy of leadership and authority within the Muslim community.

Those who became known as Sunnis believed that the leaders, or caliphs, should be chosen from among those capable of leading the community, typically through a consensus or election. Conversely, the Shi'a faction held that leadership should remain within the Prophet's family, specifically through his cousin and son-in-law, Ali, and his descendants. This fundamental disagreement on leadership criteria not only marked the political landscape of early Islam but also shaped theological and social differences that continue to influence the Muslim world today.

Other options presented, such as regional disputes over the capital city, the approach to converting others to Islam, or military strategies against external threats like the Byzantine Empire, do not capture the essence of the initial division between these two branches of Islam. They pertain to later developments and challenges faced by the Islamic state but are not at the heart of the original schism regarding leadership and authority.