A Kurdish militia with ties to an organization waging an insurgency in Turkey's southeast region violated Turkey's "red line" in Syria over the weekend by crossing the Euphrates River during an anti-ISIS operation.

The operation to take back Tishrin Dam from ISIS was staged by the Western-backed Syrian Democratic Forces and spearheaded by the Kurdish YPG — the military arm of the Kurdish Democratic Union Party (PYD).

It served as a huge blow to ISIS, which had relied on the dam to move weapons and fighters between its de-facto capital of Raqqa in Syria and the cities of Manbij and Jarablous it controls in the northern countryside of Aleppo Province.

Google Maps

But ISIS was not the only loser. The operation was also a major affront to Turkey, which declared the Euphrates a "red line" for Kurdish territorial expansion over the summer. Indeed, Turkey struck the YPG twice in October after it defied Ankara's warning not to cross the river.

So far, however, the Turks' response to the weekend incident has been relatively muted. When asked for his reaction to the Tishrin operation, Turkish Prime Minister Ahmet Davutoglu said in a news conference in Serbia that Turkey "would not look positively on Syrian forces hostile to Ankara moving to the west of the Euphrates," according to a translation by Reuters.

He added, however, that available information indicated that it was Arab forces, and not Kurds, who had crossed the Euphrates over the weekend.

This appears to be a half-truth. A small percentage of the SDF — roughly 4,000 of about 55,000 fighters — is made up of Arab groups operating as part of the SDF alliance under the joint name of the Syrian Arab Coalition. But the vast majority of SDF soldiers are more experienced fighters from the Kurdish YPG.

Aykan Erdemir, a nonresident senior fellow at the Foundation for Defense of Democracies and former member of Turkish Parliament, said he thinks Davutoglu's subdued response to the operation was his way of "accepting the inevitable."

A fighter from the Democratic Forces of Syria takes an overwatch position at the top of Mount Annan. REUTERS/Rodi Said

"A harsher response on Davutoglu's part would have been an admission of failure to guard his 'red line,'" Erdemir told Business Insider on Tuesday. "By portraying the event as crossing of the Euphrates River by Arab forces, he is attempting to reframe the embarrassing developments to make them appear less damaging."

Davutoglu, along with Turkish President Recep Tayyip Erdogan, harbor the fear that any movement west of the river might allow the Kurds to link their self-declared cantons, or territories, in northern Syria and create an autonomous Kurdish state along the Turkish border.

Indeed, the capture of Tishrin is "a huge first step for the Kurds in clearing out the remaining border strip controlled by IS along the Turkish border," Wladimir van Wilgenburg, a Kurdish affairs expert embedded in Iraqi Kurdistan, told Business Insider on Tuesday, referring to an alternate acronym for ISIS.

"The Turks were opposed to this," he added, "and it was my understanding that the US understood Turkish concerns and therefore also opposed YPG advances."

But the SDF operation, spearheaded by the Kurds, was reportedly aided by several US airstrikes west of the Euphrates near Manbij.

A Kurdish female fighter from the People's Protection Units (YPG). REUTERS/Rodi Said

Merve Tahiroglu, a research associate focusing on Turkey at the Foundation for Defense of Democracies, said Davutoglu's comments "are an example of how Turkey is able to maintain its 'red line' without appearing to completely impede the anti-ISIS coalition’s efforts along its border."

Tahiroglu added:

One worry in Ankara since the diplomatic crisis with Moscow last month has been Russian support for the [Kurdish] PYD and, in particular, a possible PYD movement toward the west of the Euphrates with Russian encouragement and air support.

Turkey has suffered a near-total defeat of its Syria policy since Russia entered the war on the side of the regime in September.

Russia's bombing campaign in the north, which escalated in the wake of Turkey's downing of a Russian warplane last month, has set the stage for the Kurds to continue advancing westward toward the strategically important city of Azaz.

That goal is even more attainable now that the YPG has crossed the Euphrates — with minimal condemnation from Turkey.

Google Maps

Turkey had been using Azaz as a corridor to funnel weapons and aid to the rebels it supports in Aleppo, but Russia's entry into the fray has dramatically limited Ankara's ability to change facts on the ground. That is perhaps one major reason why Davutoglu has tried so hard to reframe the Kurdish victory as an Arab one.

"Davutoglu is aware that he has very limited options to unilaterally intervene in the Syrian scene to back his 'red line,' so he is avoiding bold statements he can't back with action," said Erdemir, the former Turkish Parliament member.

Still, Tahiroglu said, because the Tishrin operation was supported by the US and not Russia, it was not a complete "nightmare scenario" for Turkey.

"But the question now," she added, "is who in the SDF will come to control this liberated land: the Arabs or the Kurds?"