The divide between the Sunni and Shiite branches of Islam is both ancient and still highly consequential today. In Syria, a Sunni-majority country dominated by members of a Shiite sect, fighting that began as anti-government has taken on sectarian overtones. That has spilled over to Iraq, which is Shiite-majority and has a predominantly Shiite government but is increasingly troubled by Sunni rebels. And the region's major powers have long pushed sectarian interests, with Shiite-majority Iran on one side and Sunni-majority Saudi Arabia on the other.
In this two-minute video, reporter Karen DeYoung and The Washington Post's video team give a very brief history of the Sunni-Shiite divide and what it means for Iraq's escalating violence today. It's important to note that this religious division is one of many factors driving the conflicts in the Middle East. Although theological differences are not in themselves enough to explain the fighting, it's important to understand the very basics to grasp what's happening in the region.
Here, to illustrate the Sunni-Shiite divide, is a map showing the religious groupings in the region.