AP - Syria's battle-hardened Kurds have proven their mettle against the Islamic State group, and in the process, carved out an autonomous zone across the country's north.
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But their advance has alarmed Turkey, and Ankara on Wednesday sent tanks across the border against ISIS, and demanded that the Kurds withdraw from recently seized territory.
The tensions between Turkey and the Kurds pit a NATO ally against the most effective U.S. military proxy in Syria's complex civil war.
Turkey, which is battling a Kurdish insurgency in its southeast, is increasingly concerned about an expanding Kurdish enclave just across the border, and the two sides could be on track for a confrontation.
Here is a look at Syria's Kurds:
A history of marginalization: The Kurds are a sizable ethnic group in the Middle East, inhabiting a territory stretching across what is now Turkey, Syria, Iraq and Iran. They number approximately 30 million across the region, including an estimated 2 million in Syria.
Kurds in all four countries have suffered from a history of marginalization, fueling aspirations for independence or autonomy. Syria's Kurds, who make up 10 percent of the country's population, were only granted the right to citizenship in 2011, with President Bashar Assad seeking to win over the minority in the early days of the uprising against him.
Civil war brings limited autonomy: The main Syrian Kurdish party, known as the PYD, has benefited from a tacit non-aggression pact with Assad since the early months of the war, when the Syrian army withdrew from much of northeastern Syria to battle rebels elsewhere in the country, allowing the Kurds to carve out a zone of autonomy.
The PYD's armed wing, the People's Protection Units, or YPG, later expanded that enclave by battling the Islamic State group with the aid of U.S. airstrikes. The Kurds have been driving westward over the past year and a half, and earlier this month captured the town of Manbij, a key IS supply hub west of the Euphrates River.
The Kurds now control all but 50 kilometers (30 miles) of Syria's northern frontier with Turkey.
An effective U.S. ally: The PYD, now allied with Arab fighters in a U.S.-backed grouping known as the Syria Democratic Forces, has handed IS a string of major defeats, pushing the extremists away from the Turkish border and closing in on Raqqa, the capital of the IS's self-described Islamic caliphate.
Victory on the battlefield has brought more and more Western support. Today the U.S., Britain, France and Germany all have special forces embedded with the Syrian Kurds.
The U.S. says its 300 servicemen are performing an advisory role. No other group in Syria has received even close to that level of Western support.
The PYD share control of the northeastern city of Qamishli with government forces. Last week, the party clashed with government troops in Hasakeh, a provincial capital also in the northeast, drawing Syrian airstrikes.
That in turn led the U.S. to scramble aircraft to protect American troops on the ground, in what could have resulted in the first direct U.S.-Syrian confrontation of the war. The PYD succeeded in expelling the government's forces from the city.
Angering Ankara: Turkey views the PYD as an extension of the Kurdistan Workers' Party, or PKK, which has waged a three-decade insurgency in Turkey's southeast.
Turkey also fears the rise of an autonomous Kurdish zone along its border, and has demanded the PYD to withdraw to the east of the Euphrates.
A growing list of enemies: The PYD also has tense relations with the Syrian rebels fighting to oust Assad, who view them as an extension of his government. The rebels point to Kurdish advances into rebel-held territory last spring following Syrian and Russian airstrikes.
The PYD has denied the allegations, insisting it is fighting for democratic self-rule. Turkey is a leading sponsor of the Syrian rebels.
A growing list of enemies: Riding a wave of military successes, the PYD declared in March that it would insist on a federated solution to the Syria war, alarming most of the conflict's other players who view it as plan for partition.
Both Damascus and Ankara are deeply opposed to the Kurdish proposal, and may have even found some rare common ground over it. Just days after Syrian warplanes bombed Kurdish positions in Hasakeh, Turkey announced that Assad could retain some role during a political transition in Syria, softening its previous insistence that he step down immediately.
On Wednesday, Turkish tanks and special forces poured across the border, helping a force of some 1,500 Syrian rebels seize the border town of Jarablus from IS. But while IS was the main target, the operation was widely seen as an escalation against the Kurds, who are dug in just to the south, in Manbij.
Turkey again demanded they withdraw to the east, and visiting Vice President Joe Biden said that failing to do so could mean the end of U.S. support, an indication that even Washington is concerned about the Kurds' growing strength.
Fears of betrayal: If regional powers fear a proxy run amok, the Kurds worry that they will again be used as a pawn in international statecraft.
It wouldn't be the first time.
The PKK operated freely in northern Syria under former President Hafez Assad when he was on the outs with Turkey — until 1998, when he expelled the group's revered leader as part of a thaw in relations with Ankara.
In northern Iraq, Iran's withdrawal of support for a Kurdish rebellion in the 1970s eventually left the Kurds at the mercy of Saddam Hussein, whose forces killed tens of thousands of them over the next two decades, including with poisonous gas.
The U.S., which was backing Saddam in the Iraq-Iran War, did not interfere.
Today the Kurds of northern Iraq enjoy the kind of autonomy Syria's Kurds crave. But even the Kurdish government in Irbil, which depends on good relations with Ankara to export its oil, backed Turkey's incursion into Syria. It said the PYD could not be allowed to threaten regional security.