Free Syrian Army fighters raise their flag over Tel Abyad after the border town was seized from Isis

Syria’s Kurdish forces predicted on Monday that they would need days to take control of Tel Abyad, a strategic border town long held by jihadi forces. But by nightfall, they announced the city’s capture — surprising Syrians, outsiders and perhaps even themselves.

Part of their sudden success was down to luck. Militants from the Islamic State of Iraq and the Levant (Isis) withdrew faster than expected under the force of US-led coalition air strikes. But the Kurds’ ability to exploit this advantage and swiftly surround and seize the area has driven home the sense that the People’s Protection Units, known by their Kurdish acronym YPG, may be the most successful fighting force battling Isis.

With Iraq stalled because of overstretched forces, fighting groups competing over conflicting agendas and repeated army retreats, the YPG looks like the force most capable of leading coalition-backed campaigns against Isis in Syria.

But the Kurds’ increasingly vital role in the fight against Isis also means that a population sidelined for decades in Arab majority Syria will get a bigger say in the country’s future.

“This is a big turning point for Syria. Any future Syrian state will have to accept an either fully autonomous [Kurdish] region, or it could just break away completely,” said Michael Stephens, an analyst at the Royal United Services Institute think-tank. “There is now an entity on the ground legitimised by western air power and that ultimately leads it to an inevitable separation.”

Washington is not officially in favour of more self-determination for the Kurds but Kurds and Arabs are taking its low-key stance as a green light for the YPG.

Nor has the YPG hidden its aim to link Syria’s eastern Kurdish region with Kobani to the west. The Kurdish town became a cause célèbre last year when the YPG successfully secured coalition backing to recapture it from Isis.

The YPG’s role is causing a furore among the wider Syrian opposition, which has accused the group of ethnic cleansing of Arab villages — charges that it denies.

Neighbouring Turkey, worried about its own Kurdish minority’s ambitions for autonomy, is also wary.

Haqqi Kobani, a YPG commander, said: “We’re organised. We have a single leadership. And most importantly, the YPG doesn’t retreat. We would accept death before that and so we are the ones getting the air cover.”

Many YPG leaders are veterans of Turkey’s Kurdistan Workers’ Party (PKK), which Turkey, the US and EU brand as a terrorist group. They are now fighting on their home turf, having benefited from the PKK’s three-decade guerrilla war for separation from Turkey.

Other factors have also contributed to the YPG’s most recent victory. Isis is waging several large campaigns across Syria and Iraq and may not see the border town as critical. Moreover, the Syrian-Turkish border zone, which the coalition and YPG aim to clear of Isis, is largely flat and open, making it easy to bomb.

As a group representing stateless Kurds, the YPG militants’ commitment to fight may extend only as far as there are Kurdish populations to support them. Nihat Ali Ozcan, security expert at Tepav, an Ankara-based think-tank, said the YPG cannot be a substitute for the effort to create a broader force, including Syria’s Arab rebels. “If you want to expand your influence, you should set up a strategy to control the local population,” he said.

The YPG may keep pushing until they reach Jarablus, a border town held by Isis and another mixed Arab-Kurdish area. But going further, such as south to Raqqa, Isis’s de facto capital, requires the coalition to strike a grand political bargain with Turkey, the Syrian opposition and the YPG.

“Without it, this [Tel Abyad] will be a first and last experiment in entering non-Kurdish majority areas,” Mr Mihamed said.

Even so, the victory is a game changer for Syria’s Kurds.

“It certainly gives them a stronger hand in any talks,” Mr Stephens said. “That is a shift in dynamics that is permanent.”

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