The defeat of ISIS in the northern Syrian town of Kobani last January brought Kurdish fighters well-deserved international recognition. Not only was their victory strategic, but it also struck a blow to ISIS’s image as an unstoppable force in its quest to create an Islamic state. U.S. airstrikes assisted in the battle, but credit for the success belongs to the Kurds of Syria and Turkey, aided by 150 Peshmerga from Iraqi Kurdistan (KRG).

By most accounts, the Syrian Kurds—the People’s Protection Units (YPG)—and their counterparts from Turkey, the Kurdistan Workers' Party (PKK), have been the most effective forces against ISIS. But the West is hesitant to militarily contribute anything to them beyond air support, despite repeated calls from the Kurds for arms. Instead, the U.S. has funneled weapons toward moderate Syrian opposition rebels. To bolster these forces, the U.S. and Turkish governments will soon begin implementing a plan to train and equip 5,000 fighters a year for three years—though neither government can agree whether the real enemy is ISIS or Syrian President Bashar Al Assad.

So why hasn't the U.S.-led coalition armed the YPG, given their ability and willingness to confront ISIS? Most analysts agree that Turkey is the biggest obstacle. Joshua Landis, an expert on Syria and professor at the University of Oklahoma, said in an email that U.S. hesitation to arm the Kurds stems from their “connection to the PKK and a desire not to alienate the Turks more than necessary.” Greater Kurdish autonomy in Rojava—the three Kurdish-majority regions in northern Syria—could further embolden the Kurdish rights movement within Turkey’s borders.

Aaron Stein, an associate fellow at the Royal United Services Institute and an expert on Turkey, emphasized this point. “We aren’t arming [the Kurds] because we still take some of our cues from the Turkish government,” he said. The fact that the U.S. is even offering air support to the YPG is a “big deal,” according to Stein. “The airstrikes are a drastic change from what we used to do. It’s a fantastic change from our old policy. … It did not go over well in Ankara.”

The U.S. and Turkey, which is a NATO ally, still view the PKK—which has fought the Turkish state in a bloody insurgency for some three decades—as a terrorist group, even though the group declared a unilateral ceasefire in 2013 and is currently engaging in peace talks with the government. And the YPG in Syria are closely tied ideologically and operationally to the PKK. “They are linked through the supranational structure to the PKK. … And you have PKK embedded with them quite closely,” Stein said. The legal issue of arming a group on the U.S. State Department list of Foreign Terrorist Organizations (FTOs)—where the PKK, not the YPG, is listed—is also deterring lethal aid to the Kurds. But the U.S. only just removed the two dominant political parties in Iraqi Kurdistan from the list of terrorist groups in December, despite having been allies for the last decade. Some are calling for the delisting of the PKK as well.