A painting depicts the battle of Karbala in 680, in which Imam Hussein engaged a superior Arab army and was killed in battle.
A painting depicts the battle of Karbala in 680, in which Imam Hussein engaged a superior Arab army and was killed in battle. Brooklyn Museum/Corbis
The division of Islam into Sunni and Shia branches goes far back in Muslim history to the aftermath of the death of the Prophet Muhammad. Its repercussions have rippled through history, with periods of peace and periods of war. With the recent turmoil, the conflict between Shia and Sunni is once again a driving force behind events in the Middle East. Read a chronology:
570: The Prophet Muhammad is born.
598: Ali, who will become the fourth caliph and the first Shiite Imam, is born.
610: The year Muslims cite as the beginning of Muhammad's mission and revelation of the Koran.
613: The public preaching of Islam begins.
630: The Muslims, led by Muhammad, conquer Mecca.
632: Muhammad dies. Abu Bakr is chosen as caliph, his successor. A minority favors Ali. They become known as Shiat Ali, or the partisans of Ali.
656: Ali becomes the fourth caliph after his predecessor is assassinated. Some among the Muslims rebel against him.
661: Violence and turmoil spread among the Muslims; Ali is assassinated.
680: Hussein, son of Ali, marches against the superior army of the caliph at Karbala in Iraq. He is defeated, his army massacred, and he is beheaded. The split between Shia and Sunnis deepens. Shia consider Ali as their first Imam, Hussein as the third Imam.
873: The 11th Shiite Imam dies. No one succeeds him.
873-940: In the period, known as the Lesser Occultation, the son of the 11th Imam disappears, leaving his representatives to head the Shiite faith.
940: The Greater Occultation of the 12th or Hidden Imam begins. No Imam or representative presides over the Shiite faithful.
1258: The Mongols, led by Hulagu, destroy Baghdad, ending the Sunni Arab caliphate.
1501: Ismail I establishes the Safavid Dynasty in Persia, and declares Shiism the state religion.
1900: Ruhollah Khomeini is born in Persia.
1920-1922: Arabs, both Shia and Sunni, revolt against British control of Iraq.
1922-1924: Kemal Ataturk abolishes the Ottoman sultanate and the Turkish Sunni caliphate.
1925: Reza Khan seizes power in Persia, declares himself Shah, establishing the Pahlavi dynasty.
1932: Iraq becomes an independent nation, under King Feisal, a Sunni Arab.
1935: Persia is renamed Iran.
1941: Reza Shah abdicates throne in favor of his son Mohammad Reza Shah. British and Soviet military forces occupy Iran.
1953: A joint CIA/British intelligence operation in Iran keeps the Shah on the throne and ousts nationalist Prime Minister Mohammad Mossadegh.
1963: Amid widespread protests in Iran against the Shah, Ayatollah Khomeini is arrested, then exiled to Najaf in Iraq.
1967: Israel defeats Egypt, Syria and Jordan in the Six Day War.
1968: The Baath Party seizes power in Iraq.
1973: Israel defeats Egypt and Syria in the Yom Kippur War.
1978-79: Widespread protests force the Shah to abdicate and flee Iran. Ayatollah Khomeini returns to Iran to lead the revolution.
1979: Saddam Hussein seizes power, becomes president of Iraq. Iranian revolutionary students seize the U.S. embassy in Tehran and take diplomats hostage. They are released in January 1981.
1980: Saddam Hussein orders the Iraqi army to attack Iran.
1980-1988: Iran-Iraq war. Hundreds of thousands die on each side and the war ends in a stalemate.
1982: Israel invades Lebanon, seizes Beirut. Hezbollah is formed in Lebanon.
1983: Suicide truck bombers, believed to be Hezbollah, kill 241 American servicemen in Beirut.
1989: Ayatollah Khomeini dies in Iran.
1990: Saddam Hussein orders his army to seize Kuwait.
1991: The U.S. military ousts the Iraqi army from Kuwait. Shia of southern Iraq rebel against Saddam Hussein, who puts down the rebellion brutally. Thousands of Shia are killed.
1991-2003: Iraq is placed under economic sanctions. U.N. weapons inspectors destroy most of Iraq's nuclear, biological and chemical weapons programs.
2001: Al-Qaida, led by Sunni Muslim fundamentalists, mounts attacks in the United States, killing 3,000 people. The United States invades Afghanistan and ousts the Sunni Taliban government.
2003: The U.S. military invades Iraq, topples Saddam. An Iraqi insurgency erupts, led by Sunni Baathists and al-Qaida.
2005-2006: Iraqi elections bring Shiite political parties to power in Baghdad, backed by Iran. Sunni-Shiite sectarian violence intensifies.
2005: Hard-line fundamentalist Mahmoud Ahmadinejad is elected president in Iran. Iran pursues acquisition of nuclear technology.
2006: War breaks out between Israel and Hezbollah in Lebanon. The U.N. Security Council imposes economic sanctions on Iran in response to nuclear activities.
2007: The United States sends additional troops to Iraq.