When Muhammad died in 632 without a clear successor, the question of who held the right to lead fractured the Muslim community. Sunnis accepted caliphs chosen by the community's leading men, beginning with Abu Bakr. Shia Muslims held that authority belonged to the Prophet's family — his cousin Ali and his descendants. The killing of Ali's son Husayn at Karbala in 680 crystallized Shia identity around grief and martyrdom. The split unfolded gradually into distinct theologies, legal schools, and devotional practices that still divide roughly two billion Muslims today.
When the Prophet Muhammad died in 632, he left no son who lived to adulthood and no instruction about the succession that the whole community accepted as decisive. A community that had been bound together by his presence now had to decide, suddenly, how to continue without him. The men who gathered to choose a successor that day did not know they were standing at the start of a division that would outlast every empire built in Islam's name. The question they faced was simple to ask and impossible to settle: who had the right to lead, and on what authority? Fourteen centuries later, roughly two billion Muslims still answer it in two main ways.