President Donald Trump approved a plan to directly arm Syrian Kurdish fighters in North Syria battling ISIS, according to reporting on May 9.

Pentagon Spokesperson Dana White agreed with the widespread view that Kurdish and other forces within the Syrian Democratic Force (SDF) were the sole option for defeating ISIS at this moment.

“The (Syrian Democratic Force), partnered with enabling support from U.S. and coalition forces, are the only force on the ground that can successfully seize Raqqa in the near future,” White said.

The United States took this long to commit to arming SDF, despite its stated objective of destroying ISIS, because of concerns from the Turkish government. Turkey has continually asserted, not unreasonably, that Kurdish forces within SDF have connections to the Kurdistan Workers Party, known as the PKK. Turkey has designated PKK as a terrorist group because they have been fighting a guerrilla war against the state over the oppression of the Kurdish minority within the country.

On Wednesday, Turkish President Tayyip Erdogan called on the U.S. to reverse its decision. “We want to believe that our allies will prefer to side with us, not with a terrorist organization,” he said, adding that he would use his upcoming meeting with President Trump to lobby against the program.

The group in question within the SDF, the People’s Protection Units (YPG), does have connections to the PKK. Turkey has legitimate concerns that weapons flowing to the YPG may end up in the hands of the PKK. Or, even worse, that after defeating ISIS in Raqqa and elsewhere, YPG forces will join up with PKK guerrilla fighters in Turkey.

But the Pentagon’s point is salient. Only the SDF can defeat ISIS right now and YPG makes up a significant portion of SDF forces. At the same time, Turkey’s role in the Syrian conflict raises questions as to whether they even want ISIS defeated. They have supported both ISIS and Al Qaeda with arms and by opening the border for Sunni jihadist fighters to flood Syria.

Turkey’s direct military participation in Syria, “Operation Euphrates Shield,” has produced mixed results in the fight against ISIS, although that may be because Turkey’s actual objective was to counter Kurdish influence.

In any case, it seems unlikely that President Trump, who has said he is essentially letting the Pentagon run the war, is going to reverse his decision to arm the SDF. How will Turkey respond?

Though Turkey does not appear to have enough power to stop the SDF arms program, the country has considerable leverage with the U.S. given its strategic location and that it hosts a major U.S. military base in Incirlik. Without that base, the U.S. would have considerable difficulty operating in the region.

One of the major papers in Turkey, Sozcu, has already called for the government to shutdown the U.S. base in retaliation. Sozcu is antagonistic to the Erdogan government, which suggests opposition to the U.S. arming the SDF is a widely held opinion in Turkey. President Erdogan has every incentive to play up the controversy.