Turkish army tanks and Turkish-backed Syrian rebel fighters make their way in the Syrian border town of Jarablus on Aug. 24.

Turkish army tanks and Turkish-backed Syrian rebel fighters make their way in the Syrian border town of Jarablus on Aug. 24.

Turkey sent more tanks into Syria on Thursday, one day after helping Syrian rebels drive the Islamic State out of a town just over the border.

But the operation in the Syrian town of Jarablus also has a second aim: to block the advance of a US-backed Kurdish militia that Turkey views as a threat equal to ISIS.

Turkey's “Euphrates Shield” operation marks the first time a foreign state has officially sent ground troops into Syria to support that country's armed opposition. The Syrian government condemned the offensive as a “blatant violation of sovereignty.”

Beginning Wednesday morning, the rebel fighters were able to take full control of the town by the end of the day. The town had been a key gateway for smuggling fighters and supplies from Europe and elsewhere to ISIS.

Turkish officials said the operation aimed to clear their border of terrorist groups, enhance border security and “support the territorial integrity of Syria.”

The contingent of tanks, artillery, special forces and warplanes made it Turkey’s largest intervention in Syria since the country's civil war began five years ago.

It follows months of rocket fire and terrorist attacks against Turkey by ISIS. On Saturday night, more than 50 people were killed by a suicide bombing at a wedding in the southeastern Turkish city of Gaziantep, an attack Turkey blamed on the extremist group.

On Wednesday, US officials said the Turkish forces and Syrian rebel fighters had support from the US-led anti-ISIS coalition in the form of drone surveillance and military advisers. “We are syncing up with the Turks,” one senior US official told the Wall Street Journal.

But the unity displayed between the two NATO allies was undercut by the Turkish government's pledge to contain Syrian Kurdish rebels — who are a key ally of the US in the fight against ISIS.

"At 4 this morning, operations started in the north of Syria against terror groups which constantly threaten our country," like ISIS and the Syrian Kurdish rebel group, Turkish President Recep Tayyip Erdogan said in a speech announcing the operation.

Turkey deployed more than 20 tanks in the Syrian town, and will send more as needed, a Turkish official told Reuters. The news agency said a witness saw at least nine Turkish tanks enter Syria on Thursday, and 10 other tanks waited on the Turkish side of the border.

The US has played a delicate balancing act with its partners in the fight against the Islamic State, relying on a web of alliances that's brought the bitterest of enemies on to the same side.

The US-led international coalition to defeat ISIS operates out of — and in cooperation with — Turkey, but is also backing the Kurdish People’s Protection Units, or YPG. 

Turkey considers the YPG a terrorist organization, and an offshoot of the Kurdistan Workers' Party militant group that's been fighting the Turkish state for 40 years for more autonomy and rights for Turkey's Kurds.

Read more: Pentagon denies enforcing a no-fly zone in northeast Syria. Kurdish allies say otherwise.

Officials in the capital Ankara fear that if Kurds succeed in their quest for autonomy in northern Syria, that will ignite Kurdish separatist demands in Turkey even more.

The US classifies the Kurdistan Workers’ Party as a terrorist group, but says the Syrian Kurdish militia is a separate organization. Many analysts say the distinction is not as clear as Washington might hope.

The YPG is the main component of an alliance of Arab and Kurdish rebels called the Syrian Democratic Forces. The alliance has made steady progress pushing east, capturing ISIS-held areas with support from US fighter jets and military advisers. That progress has worried Turkey, and deepened a rift between Turkish and US officials over Washington's backing for the group.

Earlier this month, the Syrian Democratic Forces drove ISIS out of the Syrian city of Manbij. The forces then set their sights on Jarablus, around 20 miles north. Capturing the town would have brought Kurdish ambitions of a contiguous stretch of land along the Turkish border — an area they hope would constitute an autonomous region — closer to reality.

The move toward Jarablus appeared to be a step too far for Turkey. 

In an effort to block the Kurds' advance, Turkey backed a mainstream Syrian rebel group that opposes Syrian President Bashar al-Assad — and that clashes frequently with the YPG — to take over Jarablus.

Turkey’s intervention earned US backing. Visiting Ankara for talks with Erdogan on Wednesday, Vice President Joe Biden joined Turkey in reining in the YPG. Biden echoed Turkey's long-standing demand in saying the YPG "must move back across the Euphrates River. They cannot, will not, under any circumstance get American support if they do not keep that commitment.”

On Thursday, the Syrian Kurdish fighters gave mixed signals to those demands. YPG spokesman Redur Xelil condemned Turkey’s “hostile approach.”

“We are in our own land and we will not leave it as per some request,” he said, according to Kurdish media outlet Firat News.

“[The] Turkish state cannot shape our position there in accordance with its own interests. Our forces will remain there and there will be no retreat. Nobody has the right to impose YPG's withdrawal from there and we will never accept such a thing."

On the same day, however, a spokesman for the anti-ISIS coalition said the “main element of [the Syrian Democratic Forces] Manbij liberation force has gone east [toward the Euphrates]; some forces remain to finish clearing, IED removal as planned.”

If the YPG complies with the demand, the prospect of clashes between Turkish-backed forces and Kurdish fighters well be dampened for now. A potentially major stumbling block for the various forces lined up against ISIS may be overcome in the short term.

Aaron Stein, a Turkey analyst and senior resident fellow at the Atlantic Council, said the new arrangement might even benefit US-Turkey relations.

“This is a net benefit for US-Turkish relations,” he told PRI on Wednesday. “The US had pledged to take Turkish security concerns into account regarding the YPG presence west of the Euphrates and has stuck to its commitment. I think this operation is the next logical move following the successful Manbij operation.”

ISIS is losing ground in Syria and Iraq. The two major cities still under its control — Raqqa in Syria, and Mosul in Iraq — are coming under increasing pressure by a vast array of forces. Even though the group is expected to put up a fierce resistance in both places, many players in the anti-ISIS coalition are looking toward a post-ISIS landscape.

In Syria, the US faces the hard task of balancing Turkey’s fears over Kurdish aspirations with supporting a vital partner in the fight against the Islamic State.

Soon, Turkish tanks could share a front line with emboldened Kurdish fighters, raising the possibility of a violent flare-up between the two.

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