WASHINGTON (NYTIMES, REUTERS) - United States President Donald Trump's decision to withdraw his troops from Turkey's border with Syria and let Turkish forces come into northern Syria instantly cast into doubt the fate of ethnic Kurds there.

The Kurds have been the US' closest allies in the fight against the Islamic State in Iraq and Syria (ISIS) group, and had worked to achieve a degree of self-rule in that stretch of Syria.

Motivated largely by the aim of containing Syrian Kurdish power, Turkey already has troops on the ground across an arc of north-western Syria, the last stronghold of anti-Damascus rebels.

Here's a look at the players in war-torn Syria and the long enmity between Turkey and the Kurds.

KURDS' ROLE IN SYRIA

The Kurds are the driver within the Syrian Democratic Forces (SDF) - a loose coalition of militias led by Syrian Kurdish fighters which came together to help the US fight the hardline ISIS militants in Syria.

SDF fighters received air support and training from the US and fought together on the ground with US Special Operations forces against ISIS militants, losing thousands of fighters.

That has won the group praise from a range of top US officials, and after eight years of the multi-sided war in Syria, the SDF remains the only significant armed group still aligned with Washington.

While many of the group's fighters and most of its leaders are ethnic Kurds, the SDF also includes Arabs and members of Syria's other religious and ethnic minorities. Its ideology is secular, and it promotes a form of democracy characterised by rule at the community level.

The US has given the SDF generous military support, but it has not endorsed the group's political project, in part to avoid alienating Turkey.

Since the official destruction of ISIS caliphate early this year, the SDF has continued to pursue ISIS remnants in cooperation with US forces while seeking to strengthen the network of local councils that have been established to govern areas liberated from ISIS.

The Kurdish forces have also become the de facto guardians of tens of thousands of former ISIS residents and jailed fighters in northern Syria, and they receive limited aid to do so.

If a new conflict breaks out in the area, the question of what happens to those ISIS prisoners and their family members will become urgent.

WHY TURKEY WANTS TO ROOT OUT KURDS

The US' close cooperation with the SDF has angered Turkey, a US ally in Nato. Turkey accuses the Kurdish fighters of being terrorists and closely linked to the PKK, a guerrilla organisation that has fought a bloody, decades-long insurgency against the Turkish state.

Both Turkey and the US consider the PKK a terrorist organisation. But US officials have publicly tried to play down the links between that group and the SDF while privately acknowledging that they exist.

While there are few clear examples of militant attacks on Turkey originating from SDF-controlled territory, Turkey has watched the growth of Kurdish autonomy across its southern border with a rising sense of alarm, fearing that it could pose a national security threat.

Turkey has often raised these concerns with the US, and in recent weeks, US officials sought to bring down tensions by brokering security arrangements near the Syrian-Turkish border with both sides.

But those measures failed to satisfy Turkish officials, prompting the country's president, Mr Recep Tayyip Erdogan, to inform Mr Trump in a phone call on Sunday (Oct 6) that he planned to send his forces into Syria to root out the Kurdish forces.

Additionally, Turkey also wants to create a space inside Syria where two million Syria refugees currently hosted in Turkey can be settled. It had been pushing the US to jointly establish a "safe zone" extending 32km into Syrian territory, but repeatedly warned it could take unilateral military action after accusing Washington of dragging its feet.

WHAT NOW FOR U.S. ALLIES?

The SDF accused the US on Monday of failing to fulfil its obligations by pulling out its troops, which paved the way for Turkey to invade.

The SDF also warned that a Turkish incursion could undo the gains made against ISIS.

It said it would "not hesitate for one instant to defend ourselves" and called on the area's people to "defend our homeland from Turkish aggression".

It was unclear on Monday when and where Turkish forces would cross into Syria, but the sense of betrayal by the US among Syrian Kurds was clear.

"US forces on the ground showed us that this is not how they value friendship and alliance," Mr Mustafa Bali, a spokesman for the SDF, wrote on Twitter on Monday, adding that Mr Trump's decision was "about to ruin the trust and cooperation between the SDF and US".

Still, the Syrian Kurds have few other supporters to turn to. Inside Syria, most of what remains of the country's rebel movement is backed by Turkey and opposes the SDF, and its relations with the government of President Bashar al-Assad are chilly.

Some analysts speculate that the Kurds will be forced to court Mr Assad's government for protection.

DO RUSSIA AND IRAN BACK TURKEY'S MOVE?

Russia and Iran, the other two major foreign powers in Syria, strongly support Mr Assad - unlike Turkey and the US which both called for him to stand down and supported rebels fighting to overthrow him.

Russia has said that Turkey has the right to defend itself, but Kremlin spokesman Dmitry Peskov said on Monday that Syria's territorial integrity must be preserved and that all foreign military forces "with illegal presence" should leave Syria.

If the US pulls out all its troops from north-east Syria, the Damascus government - backed by Russia - may try to retake control of much of the region not seized by Turkey.

WESTERN REACTION TO TURKEY'S PLAN

There has been no public support from Turkey's Western allies for its plan to settle two million Syrians - more than half of the refugees it currently hosts - in north-east Syria. The main Western concerns are that an influx of Sunni Arab Syrians into the largely Kurdish north-east would change the demographics of the region.

The United Nations regional coordinator for the Syria crisis said all sides should avoid major displacement of civilians if Turkey launches an assault.

WHAT THIS MEANS FOR ASSAD

While the territory in question is already outside Syrian government control, a Turkish incursion would mean the area switching from a non-hostile force - the SDF - to Turkey and rebels that have sought to topple Mr Assad.

Damascus has long viewed Turkey as an occupying power with designs on northern Syria. It has also at times suggested a willingness to strike a deal with the Kurds, though their last negotiations got nowhere.

WHAT THIS COULD MEAN FOR ISIS

Chaos could present ISIS with an opportunity to stage a revival and the SDF has been conducting operations against ISIS sleeper cells since capturing its final territorial foothold earlier this year.

Syrian Kurdish leaders have long warned that the SDF may not be able to continue holding ISIS prisoners if the situation was destabilised by a Turkish invasion.

The SDF is still holding 5,000 ISIS fighters of Syrian and Iraqi nationality and a further 1,000 foreigners from more than 55 other states, according to the foreign relations department of the Kurdish-led administration in northern Syria.