Turkish troops head to the Syrian border, in Karkamis, Turkey, on Aug. 27. (Halit Onur Sandal/Associated Press)

THE LATEST twist in Syria’s civil war has pitted two problematic U.S. allies against one another. Turkish President Recep Tayyip Erdogan sent tanks and troops across the border last week, along with a force of Syrian rebels, to drive the Islamic State from a key border town — thereby nominally delivering the greater Turkish commitment to the fight against the terrorist entity that the United States has long sought. But as the offensive has continued, it has become clear that Turkey’s main aim is to seize territory from another U.S. ally and vital anti-Islamic State force, the Kurdish-dominated Syrian Democratic Forces. Having initially provided air support to the Turks, the Obama administration was, by Sunday, calling their clashes with the Kurds “unacceptable” and “a source of deep concern.”

This ugly muddle, which is threatening to undermine a prospective offensive against the Islamic State’s capital, Raqqa, is the inevitable product of President Obama’s narrow and attenuated engagement in Syria. The administration has sought to mobilize and support local forces in eliminating the Islamic State without forging a consensus about what should replace it, much less a practical strategy for getting there. So U.S. Special Operations forces are deployed with Syrian Kurds, who are trying to carve out an autonomous Kurdish zone in northern Syria, while U.S. planes bomb on behalf of Turkey, which has sworn to prevent any such entity and wants to repopulate the region with Sunni Syrian refugees.

Secretary of State John F. Kerry, meanwhile, is still negotiating with Russia about a cease-fire in western Syria, a plan that would advance Moscow’s goal of strengthening and entrenching the Assad regime in Damascus. How does a peaceful and stable Syria emerge from these disparate initiatives? With less than five months remaining on its watch, the Obama White House appears to have no interest in formulating an answer.

That might be seen as a practical response to what the president regards as an intractable conflict destined to drag on for years. But as the trouble in northeastern Syria demonstrates, the absence of U.S. leadership in devising a workable political settlement has debilitating consequences even for the more limited aim of destroying the Islamic State. U.S. commanders have been describing the Syrian Democratic Forces as the most capable anti-Islamic State ground force and have been counting on its members to participate in the liberation of Raqqa. Yet now those fighters are skirmishing not only with Turks but also with Free Syrian Army units that joined Ankara’s incursion and are also U.S.-backed.

The U.S. response to this has been to pressure both sides: Washington bluntly told the Kurds to withdraw their troops from an area west of the Euphrates River that Turkey wants to fold into its buffer zone. That and a cease-fire reported on Tuesday will be at best a temporary patch. What’s needed is a broader Turkish-Kurdish settlement, including Kurdish militants in Turkey whom Mr. Erdogan once courted and now wars against, and a plan for a federal Syria not dependent on either Bashar al-Assad or Vladi­mir Putin. A decade ago, the task of catalyzing those deals naturally would have been taken up by the United States. Having renounced that role, Mr. Obama will leave chaos to his successor.