This is a huge headache for Uncle Sam
Starting last night Turkey has begun a massive shelling action against the Kurdish-held NW corner of Syria around the town of Afrin. The Turks have been massing around the SDF-held Afrin enclave for days and weeks now. They claim "retaliatory" bombardment is all they are interested in but the Kurds fear this is a prelude to an all-out ground offensive.
Some clashes between Kurdish fighters and Turkish-backed rebels have reportedly already taken place. The Kurds suspect the rebels and Turkish armed forces will try to take the town of Tal Rifat. The Kurds wrestled control of Tal Rifat from rebels in February 2016—precisely when the Syrian army itself was making gains against the rebels in this part of Aleppo province.
The Turkish attack could not have come at a worst time for the Kurds—their SDF militia is right now focused on the battle for Raqqa city against ISIS in an entirely different part of the country. Even a group of Afrin SDF fighters is in Raqqa, after they traveled there over government-held territory in a deal that saw government soldiers move across SDF-held NE Syria.
As you might expect, reports now claim that the Kurds are saying they will discontinue the Raqqa operations if the Turkish moves against Afrin do not stop. The Kurds care a lot more about their ethnic kin in Afrin than they do about Arab Raqqa. The US however, cares a whole lot about having US-backed forces dislodge ISIS from what the American media has dubbed its "unofficial capital".
The Kurds are leveraging their participation in the US-directed operation in Raqqa to exert pressure on the US to get Turkey to lay off of Afrin. They are not in a weak position vis-a-vis the US either. The US has been the only power willing to help them militarily, but likewise they are the only useable proxy the US has in Syria.
A complicating factor for the US is that it has no presence in Afrin. In fact he has no links to Afrin-based SDF whatsoever and never trained or supplied it. The US purposefully ignored it reasoning there was no point in further antagonizing Turkey over an enclave which did not border ISIS and could therefore contribute little to the fight against it.
However with Erdogan taking that to mean Afrin SDF is fair game US neglect of it is driving a wedge between Americans and the SDF instead. Ideally the US would like functional relations with NATO Turkey and Pentagon-backed Syrian Kurds both, but the two are continuously testing the relationship the US has with the other, and seem determined to force the US to pick one or the other.
The last time a Turkish-Kurdish conflagration threatened US relations with the two opposing sides Russian actions helped calm things down and mitigated the fallout for the US immensely. What will happen this time?