Sources: Nahj al-Balagha (Ali’s sermons and letters); al-Kulayni, al-Kafi (Shia hadith collection); al-Mufid, Kitab al-Irshad (on the Imams); the theology of the Usuli school.

Shia Islam (from Shi’at Ali — partisans of Ali) holds that legitimate leadership of the Muslim community belongs to Ali ibn Abi Talib (Muhammad’s cousin and son-in-law) and his descendants — the Imams. The fundamental Shia conviction: at Ghadir Khumm (18 Dhul Hijja, 10 AH / 632 AD), Muhammad explicitly designated Ali as his successor before his death; this designation was ignored by the companions who chose Abu Bakr. The three great Shia tragedies: Ali’s assassination (661 AD), the martyrdom of Husayn ibn Ali at Karbala (680 AD, Ashura), and the occultation of the 12th Imam (~874 AD). Distinctive Shia theology: (1) The Imamate — the Imam is a divinely-guided, infallible authority who provides ongoing guidance; (2) Wilayah — the spiritual authority of the Imams extends beyond legal rulings to esoteric guidance; (3) Temporary marriage (mutah) — permitted in Shia law, forbidden in Sunni; (4) Taqiyya — permissible dissimulation when under threat; (5) Intercession (shafaah) — prayers to the Imams as intercessors are central to Shia devotion. The largest Shia communities: Twelver Shia (Iraq, Iran, Lebanon, Bahrain), Ismaili (subdivided), and Zaydi (Yemen).