“Red lines” have been a fixture in Ankara’s policies toward the Kurds for decades. Blurring or shifting, thinning or thickening, decreasing or increasing, myriad red lines were drawn as Ankara grappled with the painful consequences of the Kurdish problem and sought to keep it under control instead of resolving it. The more the problem became regionalized, the more the red lines crossed borders.

Another such red line was drawn in June 2015 after the People’s Protection Units (YPG), the armed wing of the Syrian Kurdish Democratic Union Party (PYD), seized the town of Tell Abyad on the Turkish border, which allowed it to link the Kurdish cantons of Kobani and Jazira. Ankara’s red line began from the Syrian town of Jarablus, on the western bank of the Euphrates right at the point where the river enters Syrian territory from Turkey, and runs southward along the same bank. The western side of the red line was held by the Islamic State (IS), with the 90-kilometer (56-mile) border stretch from Jarablus westward as the group’s only remaining land link with the outside world.

On the eastern side of the red line, i.e., the Euphrates’ eastern bank, the Kurdish canton of Kobani was controlled by the PYD, the Syrian extension of the Kurdistan Workers Party (PKK), which Turkey is battling on its own territory.

Ankara’s red line was meant to hold back the PYD, not IS. Conversely, if IS had managed to capture Kobani, Ankara would have hardly been annoyed, as evidenced by the partial IS invasion of the city in 2014 when the Turkish leadership showed no sign of discontent. But if the PYD was to advance to the western bank of the river to oust IS from Jarablus — thus crossing the red line — Ankara would have been very much annoyed, for Turkish decision-makers opined that the PYD was there to stay and was thus more dangerous than IS, which they saw as a temporary force in the area.