Turkey has launched a military offensive against Kurdish forces in northern Syria, after the US paved the way by withdrawing troops from the area.
President Donald Trump said he would "obliterate" Turkey's economy if it did anything he considered to be "off-limits".
But the Kurdish forces, instrumental in the defeat of the so-called Islamic State (IS), have long been allies of the United States.
The decision to withdraw has been widely criticised, even by some Republicans allies. Trump's former ambassador to the UN, Nikki Haley, warned that his decision meant leaving US allies in the region "to die".
Here's a breakdown of what's going on in northern Syria:
Who are Syria's Kurds?
Kurdish forces have been a crucial ally of the US and its partners in the fight against IS in the area.
The US-backed force that controls the region, the Syrian Democratic Forces (SDF), is led by the Kurdish YPG militia.
Kurdish forces got a foothold in the north of Syria early in the civil war, while Assad's government forces focused on fighting the IS elsewhere in the country.
Their control was concentrated in three predominantly Kurdish regions — home to roughly 2 million Kurds — and their influence widened with the defeat of IS.
Kurdish leaders say their aim is regional autonomy within a decentralized Syria, not independence.
Why is Turkey worried?
Turkey says the YPG is indistinguishable from the Kurdistan Workers Party (PKK), which has been classed as a terrorist organization by Turkey, the US and the EU.
The PKK has been waging an insurgency in southeastern Turkey for 34 years as it fights for an independent or autonomous Kurdish state.
Kurds account for 15% to 20% of Turkey's population and live mostly in eastern and southeastern areas bordering Syria. Wary of separatism, Turkey views the Syrian foothold of the YPG as a national security threat.
It wants to drive the YPG away from its border and create a space inside Syria where 2 million Syrian refugees currently hosted in Turkey can be settled.
Turkey and the US have long been at odds over Syria, but the NATO allies agreed in August to set up a zone in northeast Syria along the border with Turkey.
Turkey has accused the US of moving too slowly to create the zone and they have in disagreement over how far it should extend into Syria and who should control it.
Ankara wants it to stretch 30km inside Syria and to be cleared of YPG fighters. It has repeatedly warned about launching an offensive on its own into northeast Syria, where US forces are stationed alongside the SDF.
Turkey has not revealed the scope of the initial military operation, with an official telling Reuters: “The location, time and scope for implementing the measures against security risks will once again be decided by Turkey."
What were US troops doing in the region?
US troops were in Syria to help in the fight against IS and to help their allies ensure the group didn't return.
They were also acting as a buffer between Turkey and the Kurdish forces that Turkey wants to drive away from its border.
Trump has long called for the return of US troops from Syria.
What does this mean for the Kurds?
Despite Trump's warning to Turkey not to do anything he considers "off-limits", it's unclear exactly what he would consider unacceptable.
Trump sent the following tweets following outcry from politicians in the US, including some within his own Republican party.
The Kurds and their allies were able to set up their own governing bodies as they gained a foothold in north-east Syria, while always insisting their aim is autonomy, not independence.
The SDF said it wanted stability but vowed to respond to any attack. “We will not hesitate to turn any unprovoked attack by Turkey into an all-out war on the entire border to defend ourselves and our people,” SDF spokesman Mustafa Bali said.
What happens in the longer term depends on whether the US keeps a military presence in other parts of the northeast and east.
A full withdrawal could expose the area to the risk of more Turkish advances, an IS revival, or attempts by Iranian- and Russian-backed government forces to gain ground.