The population of Iraq includes a diverse array of religious and ethnic groups, including Assyrian Christians, ethnic Turks, and Armenians. However, most analyses of Iraq’s population begin and end with its three largest groups: Kurds, Sunni Arabs, and Shiite (or Shia) Arabs, who together account for more than 95 percent of Iraq’s total population. Kurds are concentrated in northern Iraq, Sunni Arabs in the central region, and Shiites in the southern part. Iraq’s history has long been marked by conflict between these peoples. The question of whether these three groups, concentrated in different places, can long coexist as part of one united nation has become increasingly important in the aftermath of Saddam Hussein’s fall in 2003.

Sunni Arabs and Shiite Arabs share Arabic as a common language and ethnicity but are fundamentally divided by...