Turkey launched its largest military operation into Syria today, sending tanks, planes and ground forces into Jarabulus in an effort to clear the Islamic State from its last remaining frontier with Turkey. The offensive, led by Turkey-backed Syrian rebels, came days after an Islamic State-linked suicide attack in a Turkish border town. The main objective for the operation, however, was to counter recent advances of US-backed Syrian Kurds along the Turkish border.

Above all, it was a message: a reminder of Ankara’s determination to limit US cooperation with the Kurds, Washington’s most effective partner against the Islamic State in Syria.

Turkey has been preparing for the Jarabulus operation for days. Last weekend, an Islamic State-linked suicide bomber attacked a Kurdish wedding in the southern Turkish city of Gaziantep, killing 54 – many them children. The attack was the latest in a string of Islamic State-linked bombings that have killed hundreds in Turkey over the last year. But it triggered a much stronger response than previous assaults. On Aug. 22, Turkey vowed to “completely cleanse” the Islamic State from its border region – referring to Jarabulus, the only remaining Islamic State territory at the Turkish frontier. Yesterday, the Turkish military began firing artillery into northern Syria and shelled Islamic State targets 224 times. This morning, Syrian rebel fighters crossed into Jarabulus from Turkey, and with Turkish and US air support, launched operation “Euphrates Shield” to liberate the city from the Islamic State.

But the offensive was not merely an effort to cleanse the Islamic State from Turkey’s border. Jarabulus has been under Islamic State control since January 2014, and has until recently served as a major route for foreign fighters seeking to join the terror group. For years, Ankara turned a blind eye as smuggling of militants and materials to the Islamic State continued from the border. While Turkey stepped up its border security amidst devastating terror attacks in the last year, Ankara was unwilling to carry out a military operation across the border. Notably, Turkey’s diplomatic fall out with Russia in November 2015 was a major deterrent against Turkish incursions into Syria, until Ankara and Moscow restored relations this June. But the primary trigger for today’s offensive was a recent Kurdish victory against the Islamic State in a strategic town along the Turkish border.

The Syrian-Kurdish People’s Protection Units (YPG) is a key US ally in the fight against the Islamic State, often acting as the effective ground force for the US-led coalition’s operations in Syria. But it is also the Syrian wing of the Kurdistan Workers’ Party (PKK), a Kurdish separatist group leading an insurgency in Turkey – a designated terrorist organization by Turkey and the US. Since 2014, Washington’s military cooperation with the YPG in Syria has incensed Ankara, which sees the YPG’s territorial expansion along the Turkish border as a major security threat. As a compromise, the US has built an umbrella organization, the Syrian Democratic Forces (SDF), which includes both Arab and YPG forces. And Turkey has largely tolerated US cooperation with SDF, despite its Kurdish dominance. But Ankara also declared a red line: it would not allow the YPG to cross to the west of the Euphrates River. (The Kurds controls three unconnected cantons in northern Syria along the Turkish border, which they could unite by crossing the river.)

This month, the YPG crossed that red line. In a US-backed military operation against the Islamic State in Manbij, the SDF scored a major victory against the jihadi terror group and cut off one of the Islamic State’s most important supply lines. The conquest also meant that the Kurds were now poised to move against the Islamic State in Jarabulus. To preempt such an advance, Turkey had to launch its own operation, with its own rebels of preference, to liberate the town from the Islamic State. Indeed, Turkish President Recep Tayyip Erdogan could not have been clearer in his remarks on today: “Turkey will take matters into its own hands if required to protect [Syria’s] territorial integrity,” he said in a speech in Ankara. He was referring to the Kurds’ efforts to establish autonomy within northern Syria.

Clearing the Islamic State from the Turkish border is as much a win for the United States as it is for Turkey, and operation Euphrates Shield reportedly employed US intelligence and air support. But that support also risks undermining Washington’s military relationship with the Kurds, who have contributed more to the coalition’s battle against the Islamic State in Syria than any other actor, including Turkey. With the Jarabulus operation, Turkey has reminded both the US and the Kurds that its military is prepared to enforce its red lines. And by supporting the operation, Washington has reminded the Kurds that Ankara is still an ally.

Crucially, during his visit to Turkey today, Vice President Joe Biden declared that the Syrian Kurds would “lose US support if they did not retreat to the east bank of the Euphrates.” The promise was Washington’s most powerful public reassurance to its embittered NATO partner to date. To the Kurds, it was a slap in the face.

To be sure, the Jarabulus operation came at an exceptionally sensitive time in US-Turkish relations – one that certainly played to Turkey’s advantage. Bilateral ties have suffered greatly since a failed coup attempt in Turkey in June, in which some Turkish officials have alleged US complicity. The Turks have identified a US-based Turkish cleric as the main culprit, and demanded his extradition. Anti-American sentiment in Turkey is at its peak. Moreover, high-level bilateral meetings with Russia and Iran since the abortive coup have stirred suspicions of Ankara moving closer to Moscow at the expense of its partnership with Washington. Vice President Biden’s trip to Turkey today was primarily aimed at smoothing relations by showing US solidarity with Ankara.

But in Syria, differences between the two NATO allies run deep and are likely to persist. Despite today’s cooperation against the Islamic State, Turkey and the US are far from seeing eye-to-eye on the future of Syria, or on picking the right partners on the ground. Washington continues to rely on the Kurds in its fight against the Islamic State. Turkey continues to support Islamist groups, and turn a blind eye to the jihadists. Reports of some jihadists’ involvement in the Jarabulus operation today will only exacerbate U.S. concerns about any proactive role for Turkey in the Syrian conflict. For now, operation Euphrates Shield appears to have strengthened Turkey’s hand against the Kurds. But Washington’s cooperation could prove just as tactical as has been its support for the Kurds.

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