The Sunnis, however, regard Ali as well as the three caliphs before him as rightly guided and themselves as the true adherents to the Sunnah, or the prophet’s tradition. Sunni rulers embarked on sweeping conquests that extended the caliphate into North Africa and Europe. The last caliphate ended with the fall of the Ottoman Empire after World War I.

How do their beliefs differ?

The Sunni and Shiite sects of Islam encompass a wide spectrum of doctrine, opinion and schools of thought. The branches are in agreement on many aspects of Islam, but there are considerable disagreements within each. Both branches include worshipers who run the gamut from secular to fundamentalist. Shiites consider Ali and the leaders who came after him as imams. Most believe in a line of 12 imams, the last of whom, a boy, is believed to have vanished in the ninth century in Iraq after his father was murdered. Shiites known as Twelvers anticipate his return as the Mahdi, or Messiah. Because of the different paths the two sects took, Sunnis emphasize God’s power in the material world, sometimes including the public and political realm, while Shiites value in martyrdom and sacrifice.

Which sect is larger, and where is each concentrated?

More than 85 percent of the world’s 1.5 billion Muslims are Sunni. They live across the Arab world, as well as in countries like Turkey, Pakistan, India, Bangladesh, Malaysia and Indonesia. Iran, Iraq and Bahrain are largely Shiite. The Saudi royal family, which practices an austere and conservative strand of Sunni Islam known as Wahhabism, controls Islam’s holiest shrines, Mecca and Medina. Karbala, Kufa and Najaf in Iraq are revered shrines for the Shiites.

Saudi Arabia and Iran, the dominant Sunni and Shiite powers in the Middle East, often take opposing sides in regional conflicts. In Yemen, Shiite rebels from the north, the Houthis, overthrew a Sunni-dominated government, leading to an invasion by a Saudi-led coalition. In Syria, which has a Sunni majority, the Alawite Shiite sect of President Bashar al-Assad, which has long dominated the government, clings to power amid a bloody civil war. And in Iraq, bitter resentments between the Shiite-led government and Sunni communities have contributed to victories by the Islamic State.