Article one treats sectarianism mostly as a sociological category and a general pattern of division. Islam's central division supplies the clearest case of how a single historical dispute, argued over centuries, hardens into distinct legal and theological traditions rather than remaining a passing disagreement.
The split originates in the question of legitimate leadership after Muhammad's death in 632. The party who became the Sunni majority accepted a sequence of caliphs chosen by the early community, beginning with Abu Bakr. The party who became the Shia held that leadership belonged by right to Ali, Muhammad's cousin and son-in-law, and his descendants, rejecting the first three caliphs' legitimacy. The dispute turned violent and definitional at Karbala in 680, where Ali's son Husayn was killed by Umayyad forces; his death is commemorated annually on Ashura, especially within Shia practice, as the tradition's central act of mourning and identity.
The dispute hardened well past a disagreement about one succession into distinct systems: Twelver Shia doctrine holds that twelve infallible imams descended from Ali carry unique religious authority, the last in occultation and expected to return, while Sunni Islam built its authority structure around legal schools and consensus rather than a line of designated successors. Today the division is frequently read through the regional rivalry between Shia-majority Iran and Sunni-led Saudi Arabia, which regional scholars generally treat as a case where genuine doctrinal difference does far less explanatory work than politics and state competition, echoing article one's caution against reading such conflicts primarily off their theology.