(New York Times) -- Desperate for a strong regional ally in the fight against ISIS, both the Trump and Obama administrations eagerly worked with Kurdish forces in Syria, even though allies of those forces were waging an insurgency across the border in Turkey, a NATO ally.

Now, American successes against ISIS are threatened by Turkish attacks on the Syrian Kurds. The clash, long feared, could provoke a wider war and a division of Syria into zones of influence. But stopping it would require a diplomatic commitment on all sides that has so far been lacking.

The confrontation began last week when Turkish forces crossed the border into Syria and attacked Kurdish troops, who control the northwestern town of Afrin. Since then, events have escalated, with President Recep Tayyip Erdogan of Turkey threatening to take the fight to the Kurdish-controlled town of Manbij, where American Special Operations forces are based. He is also talking about expelling the Kurds and resettling the area with Syrian refugees.

The Kurds, known as the People’s Protection Units, dominate the Syrian Democratic Forces, an umbrella organization that receives American training, weapons and air support. The Turks consider them terrorists, indistinguishable from the Kurdistan Workers’ Party that has waged a separatist war in Turkey for more than 30 years. While the Syrian group plays down its ties to the organization in Turkey, and there are differences, links do exist. The two groups have common roots, but experts say the Syrian contingent has largely kept a 2012 promise not to provide material support to the Turkish Kurds, who are formally recognized as a terrorist group by the United States and Europe.

Despite the complications of Americans working with one Kurdish group while another group waged an insurgency against a NATO ally, the decision made sense. Turkey was so focused on overthrowing President Bashar al-Assad of Syria that it refused to help America fight ISIS. It left its border wide open, enabling foreign fighters to swell ISIS’ ranks. The Kurds, on the other hand, wanted to fight ISIS, which threatened their forces, and they were good at it.

Still, the United States should have done more to prevent the two parties from going to war with each other even before the ISIS threat receded.

Turkey had grown alarmed about American collaboration with Kurdish forces after 2012, when those forces created an autonomous semi-state in northeastern Syria. The Turks now want to prevent the Kurds from linking three enclaves along the border into a unified Kurdish region. They worry that such a consolidation will embolden Kurdish fighters in Syria to aid the Kurdish party within Turkey.

Those fears are understandable, but Mr. Erdogan is making things worse by attacking the Kurds in Syria, which could provoke a surge of Kurdish nationalism in the region. The offensive is part of his long-planned strategy to rally domestic support ahead of the 2019 elections, which relies heavily on portraying the United States as an enemy. It began after the Pentagon revealed plans for a new American-backed, 30,000-member border force in Syria that Turkey views as an attempt to create an autonomous Kurdish enclave.

When Turkey reacted angrily, the White House disavowed the plans and hinted it was easing support for the Kurds, but the Pentagon said a Kurdish-led force was still in the works.

To placate Turkey, Washington gave a green light to its offensive against Afrin, claiming that the Kurds in that area were not American allies. But it warned against an incursion into Manbij, where the Turks could come into direct contact with American forces.

Such mixed messages sow confusion and do little to prevent further conflict, reassure the Turks or support the Kurds.

Still, the administration needs the Kurds. Both the Trump and Obama administrations always assumed that if they gave Mr. Erdogan a free hand to wage war against the Kurds in Turkey, he would give America a free hand to work with the Kurds in Syria. That assumption was blindly optimistic, and it has left Washington with two allies at war — to the benefit of Mr. Assad as well as the remnants of ISIS.