Turkey is a longstanding U.S. ally with very legitimate Kurdish counterterrorism concerns. But since at least 2003, the U.S. alliance with the Kurds has also been critical. This informs a complicated but critical U.S. balancing act between the two sides.

President Trump slashed that balance on Monday by pulling the U.S. military out of northern Syria. It’s a grievous decision that will enable Turkey’s slaughter of innocent civilians, energize Kurdistan Workers’ Party terrorism, and endanger American security. To best understand this situation we must first consider the U.S.-Turkey-Kurdish relationship.