The cost of betraying Syria’s Kurds

I met many young Syrian fighters of all ethnicities, who under Kurdish leadership were determined to liberate their lands from Islamist despotism. In Raqqa and along the Euphrates I witnessed firsthand as steady trains of field ambulances carried Kurdish casualties in battle after battle. The Kurdish-led civil administration does the heavy lifting of guarding hundreds of ISIS’ most dangerous foreign fighters while their home countries drag their feet on extradition.

The West owes them a debt for the price they’ve paid. Instead, a U.S. departure would threaten them with disaster. Already Mr. Erdogan has directed two invasions of Syrian border regions—in 2016 north of Aleppo and this January in the northwestern Afrin region. Mr. Erdogan labels America’s Syrian Kurdish partners “terrorists,” links them with separatist rebels in Turkey, and suggests resettling their land with Arab refugees from elsewhere in Syria.

The Kurds have earned a reputation for fighting bravely, but without U.S. air power their prospects against a modern army with a robust air force would be grim. An invasion would force Kurdish forces to pull back from the front lines against the remnant of ISIS, allowing the jihadists to regroup and proliferate. It would likely spawn a fresh humanitarian catastrophe, including areas that have been mostly spared the worst of Syria’s civil war.