What caused the schism in Islam?

The schism that led Muslims to split into two main denominations, Sunni and Shia, dates from the 7th century — it is almost as old as the religion itself. It is based on a succession dispute that erupted in Medina, the capital of the first Islamic state in the Arabian peninsula, over who should lead the Muslims after the death of the Prophet Mohammed in 632 AD.

The Shia support Ali ibn Abi Taleb, the prophet’s cousin and son-in-law, who was initially passed over for the succession. Ali eventually became the fourth caliph and ruled from Medina for five turbulent years until he was killed in 661. During his reign, he faced a challenge from Muawiya ibn Abi Sufyan, the founder of the Umayyad ruling dynasty, who succeeded him as caliph.