The roots of the current violence go at least as far back as Iraq’s 2006-2007 civil war, which didn’t so much end as get put on hiatus. The spate of sectarian violence pitted the Shiite-majority government against Sunni militias and al-Qaeda in Iraq (a group from which ISIS emerged). The U.S. troop “surge” halted the bloodshed and got Sunni groups to side with the government against foreign jihadists. But it failed to produce a greater political resolution. With the departure of American forces from the country in 2011, these grave tensions reemerged.

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