A constructive deal to stabilize northern Syria.

By Wall Street Journal’s Editorial Board

The U.S. relationship with Turkey has careened from bad to worse, so it’s a sign of progress that the two sides agreed this week on a road map to secure a contested area in northern Syria. The plan will prevent the NATO allies from shooting at each other and could lead to a more coherent Syria strategy, if President Trump wants one.

The agreement Monday between Secretary of State Mike Pompeo and Turkish Foreign Minister Mevlut Cavusoglu centers on Manbij, a former Islamic State stronghold near the Turkish border. The U.S.-backed Syrian Democratic Forces retook the town from jihadis after heavy fighting in August 2016. The victory was a turning point in the war against the caliphate, which had used Manbij as a base to plan attacks on Europe and supply fighters to Raqqa.

The Turks tolerated the offensive but wanted the U.S.-backed Kurdish Peoples’ Protection Units, or YPG, to retreat east of the Euphrates River once the fighting was finished, given the YPG’s links to Turkish separatist groups. Instead, the Kurds declared a semiautonomous state and tried to unite with a bigger Kurdish enclave of Afrin to the west. Turkish strongman Recep Tayyip Erdogan sent in tanks.

The Trump Administration inherited this mess and deployed U.S. Army Rangers to Manbij last year to prevent the Turks from killing the U.S.-backed forces who had fought so valiantly against Islamic State. But that was always a precarious solution. Mr. Erdogan’s campaign against Syria’s Kurds is as much about whipping up domestic nationalism as it is rooting out Turkish separatists, so he has incentive to keep fighting. Mr. Trump’s indecision over Syria also hasn’t given the Kurds much reason to follow Washington’s lead.

Turkey is holding elections this month, and Mr. Erdogan can tout the U.S.-Turkey deal as a victory since it envisions the Kurds retreating to areas east of the Euphrates River, and largely hands Manbij to local authorities with U.S. and Turkish oversight. The Trump Administration is showing some goodwill that it can use to press Ankara to cooperate to push back on Iranian and Russian influence in Syria.

The larger question is what Mr. Trump is willing to do to end the Syrian civil war. So far his Administration has prioritized the fight against Islamic State and allowed Tehran and Moscow to build up a military presence that will be hard to dislodge. A U.S.-Turkey united front, combined with help from Israel, could be the beginning of a more fruitful approach to stabilizing the country.