Islam divides into two main sects - Sunnis (followers of the Sunna, or traditions: 85-90 per cent) and Shias (followers of the Shiat Ali, or "Party of Ali": 10-15 per cent). Often compared to Catholic-Protestant split in Christianity, but has not been as divisive or bloody - until now.
656: Twenty-four years after Muhammad's death, a crisis over succession leads to Sunni- Shia rift. Sunnis accept rule of elected caliphs, while Shias recognise only imams, descendants of the Prophet through his daughter Fatima and his cousin and son-in-law Ali.
680: Prophet's grandson Hussein is killed by Sunni forces at Karbala, a martyrdom mourned each year by Shias on the day of Ashura.
874: Muhammad al-Mahdi, Shia "Twelfth Imam", disappears in Samarra, Iraq. "Twelver" Shias believe he will reappear in the "last days" to re-establish Islam throughout the world.
16th century: Shia Persian Safavid dynasty battles Sunni Ottomans for control of Iraq - Sunni militants in Iraq today describe Shia opponents as "Safawis".
1979: Shia Islamic revolution in Iran fails to set off feared Sunni-Shia conflict in Middle East.
1980-88: Iran-Iraq war. The west supports Saddam Hussein's Sunni Iraq against Shia Iran.
1985: Hezbollah formed in Lebanon, with Iranian support, to defend Lebanese Shia interests.
2003: Sectarian conflict in Iraq worsens Sunni- Shia divide and threatens to spread.
2006: Saddam Hussein executed on the first day of Sunni Eid festival, exacerbating tensions.
Sunni and Shia beliefs are the same in principle, but are sometimes applied differently: Shia scholars have more latitude to interpret the Koran. Some doctrines - including taqiyya (concealing one's faith) and nikhat mut'aa (temporary marriage) - are Shia only.
Balance of power:
Arab leaders usually come from the "orthodox" Sunni sect, even in countries with sizeable Shia populations, such as Iraq, Lebanon and Bahrain. Shias often suffer discrimination and repression.
The wealthy and militarily powerful Shia Iran provides a counterweight to Sunni dominance of the region. Since the 1979 revolution, Sunni rulers have feared Iran will encourage unrest in their own domestic Shia populations.
Shia ascendancy in Iraq marks the first time Shias have been politically dominant in a key Arab country, worsening these fears.
Jordan's King Abdullah has warned of a "Shia crescent" stretching from Iran through Iraq to Syria and Lebanon, destabilising the Gulf; also accuses Tehran of "buying" a stake in Palestinian negotiations by sponsoring Hamas. Egypt's President Mubarak has accused Shias of being more loyal to Iran than to their own states.
Iraq provides a potential focus for sectarian war across the Middle East - Sunnis target Shia mosques and shrines. Anti-Shia paranoia is sweeping the region, spread by mobile-phone video clips allegedly showing Shias attempting to convert Sunnis.