The split is about succession, not doctrine at the outset: when the Prophet died in 632 without an undisputed arrangement, the majority accepted Abu Bakr as caliph while a party held that Ali, the Prophet's cousin and son-in-law, had been designated at Ghadir Khumm. Sunnis read that episode as an expression of affection; Shia read it as investiture. Everything else — a different hadith corpus, a different legal school, a different devotional temperature — grew from that fork.