It is no secret that the Aug. 24 Turkish military incursion into Jarablus, which sits less than 30 kilometers (roughly 19 miles) from the Turkish-Syrian border, was secondarily about the Islamic State (IS) and primarily about checking the advances of the People's Protection Units (YPG), the armed wing of the Kurdish Democratic Union Party (PYD) in northern Syria, as Cengiz Candar recounts this week.

Metin Gurcan, citing sources in Ankara, reports that IS “had been withdrawing from Jarablus for two weeks. The YPG had been preparing to move north to capture Jarablus, but Ankara pre-empted the move through [Operation] Euphrates Shield. Ankara's action is further confirmation that the true target of the operation is not IS, but to block YPG’s domination of northern Syria.”

The Obama administration has so far managed to prevent a major escalation over the Turkish-Kurdish fault line in Syria, which is no small achievement. The YPG is the backbone of the Syrian Defense Forces (SDF), the most effective armed group battling IS. The Syrian Kurdish group has, however, in recent weeks sought to press its gains, including in Hasakah, where Russian mediation walked back a confrontation between Syrian government and YPG forces, as well as in Manbij and Jarablus, where the SDF has staked out territory west of the Euphrates, which has alarmed Ankara.

Syria’s Kurds are picking a poor time to test the limits of their partnership with the United States. Conspiracy theories have circulated in Turkey of alleged US complicity in the failed military coup against Turkish President Recep Tayyip Erdogan last month. US-Turkey relations, already under strain, therefore required some diplomatic triage. During an Aug. 25 visit to Ankara, US Vice President Joe Biden fully backed the Turkish operation in Jarablus, stressed that the US supports a “united Syria” with “no separate entity on the Turkish border” and laid out a US red line that should have cheered his Turkish hosts when he said, "We have made it absolutely clear to the elements that were part of the Syrian Democratic Forces, the YPG that participated, that they must move back across the river. They cannot, will not, and under no circumstance get American support if they do not keep that commitment, period."