The Syrian Democratic Forces (SDF) leading the fighting in Raqqa are led by Kurdish People's Protection Units (YPG), with mostly Kurdish fighters and some Arab fighters. The US considers the SDF the best fighting force in the region to defeat ISIS. Turkey has never hidden its opposition to using the YPG, which is linked to the Kurdistan Workers Party (PKK), which is recognized as a terrorist group by the US and European countries. The PKK have conducted a 30-year separatist insurgency in Turkey, and in recent years have conducted some horrific terrorist attacks in Turkish cities. Turkey would have preferred to use Free Syrian Army (FSA) forced backed by Turkey for the Raqqa operation, as well as the Mosul operation in Iraq. US forces have been supplying advanced weapons to YPG fighters for the Raqqa battle. Turkey says that the US promised that the US would take the arms back after the battle ended, so that the weapons could not be used against Turkey. Now Turkish officials are furious that the US is apparently reneging on its promise to retrieve the advanced weapons.
Even worse, from Turkey's point of view, U.S. Defense Secretary Jim Mattis has left open the possibility of the opposite occurring: That the US will continue to supply the YPG with advanced weapons and equipment, even after Raqqa has been recaptured from ISIS.
Mattis said that some weapons would be recovered, but when asked about further arming of the YPG, he said, "Well, we’ll see. It depends what the next mission is. I mean, it’s not like the fight’s over when Raqqa’s over."
The criticism by Turkey's president Recep Tayyip Erdogan has been extremely harsh, complaining that it's a violation of the Nato treaty:
"We will be together in NATO, and you will act together with terrorist groups. What kind of business is this?
In this case, the NATO treaty should be revised.
Those who think that they can fool Turkey by saying that they will get those weapons back will eventually understand the vital mistake they made, but it will be too late. We will call to account the real owners of those weapons for every drop of blood they shed with those weapons." Reuters
Turkey points out that there have been a series of US provocations against Turkey. They claim that US officials have repeatedly lied about YPG and PKK support in Syria. The US promised to withdraw the YPG/PKK from the Syrian city of Manbij after ISIS was defeated, but the US handed control of the city over to them anyway. After that, according to Turkish news reports, it became clear these promises were mere diversions.
Turkey is becoming concerned that the YPG/PKK are controlling larger and larger regions of northern Syria, along the border with Turkey. Last year, Turkey launched Operation Euphrates Shield, a military operation by the FSA backed by the Turkish military, to prevent the Kurds from taking control of the entire northern border of Syria, and declaring an independent Kurdish state of Rojava.
There have been reports that Turkey is planning further military action against the Kurds in northern Syria.... Daily Sabah (Ankara) and Yeni Safak (Ankara)