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.

Data source: The Gulf/2000 Project and United Nations ReliefWeb (The Washington Post)

As you can see, Sunni and Shiite are spread out enough that they have to coexist within their respective countries, typically with one group in a majority and the other a minority. But they're also clustered enough that groups of Sunni and Shiite can develop local power bases that can compete with formal government authority. It's not ideal.