An Islamic State attack at dawn on Tuesday killed some two-dozen people in a Syrian town on the Iraqi border. Many of the victims were refugees who were fleeing ISIS-held territory in Iraq and Syria on their way to Kurdish-held Hasakah, Kurdish fighters from the Syrian Democratic Forces told reporters. The SDF and US soldiers who support them are in the midst of an offensive to take Raqqa, ISIS’s Syrian capital, and have recently made significant gains against the extremists in Tabqah. However, recent attacks by Turkey against Kurdish areas in Syria have threatened to distract attention from the offensive against ISIS.On April 25, Turkey launched air strikes against Kurdish positions at Sinjar Mountain in northern Iraq and at Karachok Mountain in northeastern Syria. Turkey claimed it targeted the Kurdistan Workers’ Party, or PKK, which it views as a terrorist group and has repeatedly asserted is working with the People’s Protection Units, or YPG, in Syria, against which Turkey appeared to threaten further action.For the US this is a red line. The YPG is part of the SDF, with which the US has partnered in the war against ISIS. US forces on the ground have cultivated a close relationship with the Kurds in Syria over the last two years. Department of State spokesman Mark Toner said on April 25 that the US was “deeply concerned” about the Turkish air strikes, which he said were made “without proper coordination either with the US or the broader global coalition to defeat ISIS.” Toner said the strikes caused a “loss of life of our partner forces” and that the safety of coalition personnel must be ensured.On April 30, after visiting the site of Turkish air strikes in Syria, the US sent its forces to patrol alongside the YPG – flying US flags – on the border with Turkey. The decision to display the colors and patrol along the border was intended by its visible show of force to deter further Turkish attacks. The US did the same thing in early March, around the northern Syrian town of Manbij. The SDF took Manbij from ISIS in 2016, but Turkey threatened to attack the town in March alongside its Syrian-rebel allies. The US flag-waving patrols deterred Turkey in Manbij and the tactic appears to have deterred Turkish forces again.The deeper meaning of the patrols is, the US is warning off its older ally in favor of its Kurdish relationship. Turkey and the US have 70 years of close relations, formed during the Cold War. But the war on ISIS has led the US defense establishment to conclude that the best bet to defeat ISIS lies with Kurdish forces and the SDF. The Turks have a different agenda which focuses on the PKK and its affiliates. Turkey has often accused the YPG of being in the same terrorist category as is ISIS. The Turkish view sees every step toward Raqqa by the SDF and the Americans as empowering the YPG, . While career diplomats in the State Department and CIA may prefer their traditional relationship with Turkey, the US Defense Department – and those who listen to it in the White House – have settled on defeating ISIS the fastest way possible. That means defending the Kurdish region from air strikes so its forces and allies can focus on Raqqa. Nothing would be more disastrous for the US than a war between Turkey and the YPG while ISIS gets breathing space to carry out attacks as it did on Tuesday.Turkey will continue to challenge US policy in Syria in the coming months and try to find allies in Washington who will listen to its point of view. Turkey views a permanent US presence in northeastern Syria as highly problematic and a provocation against its sphere of influence. At the same time, the US must decide if its relationship with the Kurds in Syria is merely one of convenience – until ISIS is defeated – or if it will build on it in the coming years.