The United States is unexpectedly pulling its troops from the border between north-eastern Syria and Turkey, raising questions over the fate of the area.
It will allow the Turkish military to launch an operation there against a Kurdish-led militia alliance that Western powers relied on to defeat the Islamic State (IS) group.
We’ve boiled down why it matters.
Why is Turkey planning an assault?
One main reason: Turkey considers the biggest militia in the Kurdish-led alliance a terrorist group. It says it is an extension of a Kurdish rebel group fighting in Turkey.
Turkish leaders want a 32km (20-mile) deep “safe zone” along the Syrian side of the border clear of Kurdish fighters. They also hope to resettle up to 2 million Syrian refugees currently living in Turkey there.
The Kurdish-led alliance says it will defend its territory and that the US is “leaving the area to turn into a war zone” and risking the re-emergence of IS.
Turkey has vowed to push back from its border members of a Syrian Kurdish militia called the People’s Protection Units (YPG).
Turkish leaders view the YPG as an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in Turkey for three decades.
The YPG dominates an alliance of Kurdish and Arab militias called the Syrian Democratic Forces (SDF), which has driven IS out of a quarter of Syria over the past four years with the help of air strikes by a US-led coalition.
Turkey has condemned the US for supporting the YPG and has carried out two cross-border offensives against the Kurdish forces.
In 2018, Turkey attacked the Kurdish-controlled of Afrin, in western Syria. Dozens of civilians were killed and tens of thousands displaced.
That December, with IS close to defeat, President Donald Trump said the US would begin withdrawing its troops from Syria. When commanders and allies expressed concern about the fate of the Kurds, he vowed to “devastate Turkey economically” it attacked them and proposed a “20-mile safe zone” along the border.
Mr Trump later suspended the withdrawal, but Turkish President Recep Erdogan continued pressing for a safe zone.
In August, the US and Turkey agreed to establish one together. Kurdish officials expressed support and the YPG dismantled border fortifications.
But two months later, Mr Trump decided to let Turkish troops set up the safe zone alone.
Mr Erdogan is confident his proposed 480km-long corridor will ensure Turkey’s border security and become home to between 1 and 2 million Syrian refugees.
Turkey will also take responsibility for all captured IS militants there, according to the White House.
The SDF says it has been “stabbed in the back” by the US, and warns that a Turkish offensive will create a “permanent warzone” and reverse the defeat of IS.
Why is Turkey worried about Syria’s Kurds?
It feels threatened by the People’s Protection Units (YPG), the military wing of the Syrian Kurdish Democratic Union Party (PYD).
The Turkish government insists the YPG is an extension of the Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in Turkey since 1984 and is designated as a terrorist group by the US and EU.
The YPG and PKK share a similar ideology, but they say they are separate entities.
The US also rejects Turkey’s portrayal of the militia, which has avoided taking sides in Syria’s eight-year-old civil war but been a key ally of the West in the battle against IS.
The YPG is the dominant force in an alliance of Kurdish and Arab militias called the Syrian Democratic Forces (SDF). With the help of air strikes, weapons and advisers from a US-led multinational coalition, SDF fighters captured tens of thousands of square kilometres of territory in north-eastern Syria from IS.
An autonomous administration has been set up in the region, thought to be home to between 500,000 and 1 million Kurds and at least 1.5 million Arabs.
Has Turkey taken any action in the past?
Despite being part of the US-led coalition against IS, Turkey has vehemently opposed supporting the SDF and tried to stop it taking control of Syria’s northern border.
In 2016, the Turkish military supported an offensive by allied Syrian rebel factions that drove IS militants out of the key border town of Jarablus and stopped SDF fighters moving west, towards the Kurdish enclave of Afrin.
The US dissuaded Turkey from also attempting to take the mainly Arab town of Manbij by force at the time, but Turkish leaders continued to insist that SDF fighters withdraw and it remained a flashpoint.
In January 2018, after US officials said they were helping the SDF build a new “border security force”, Turkish troops and allied Syrian rebels launched an operation to expel YPG fighters from Afrin.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said almost 300 civilians were killed in the eight-week battle, along with 1,500 Kurdish militiamen, 400 pro-Turkish fighters and 45 Turkish soldiers. At least 137,000 civilians fled their homes.
What prompted the talk of a “safe zone”?
In December 2018, President Donald Trump declared that IS had been defeated and announced he had ordered the 2,000 US troops helping the SDF in Syria to start withdrawing immediately.
Foreign allies and senior Republicans disputed the claim about IS and expressed concern about what might happen to Kurdish forces without US protection. Days later, Mr Trump tweeted: “Will devastate Turkey economically if they hit Kurds. Create 20 mile safe zone…”
Although the US withdrawal was delayed, Turkish President Recep Erdogan seized on Mr Trump’s proposal and said Turkish forces were ready to set up a safe zone.
The issue returned to the fore after the SDF captured the last pocket of territory held by IS in March 2019.
In August, the US military said it had agreed a framework with the Turkish military to address Turkish security concerns. The framework included a “security mechanism” in an area along the Turkey-Syria border, it added, avoiding the mention of a “safe zone”.
The US subsequently confirmed the YPG had begun withdrawing heavy weapons from border and dismantling fortifications. The Kurdish-led administration said the step showed its commitment to the security mechanism.
It therefore came as a surprise when the White House said on 6 October that, following a call between Mr Trump and Mr Erdogan, US troops would be withdrawing from the border area ahead of a Turkish operation.
It added that the US would not support the operation and that Turkey would “now be responsible for all [IS] fighters in the area captured over the past two years”. The SDF is holding 12,000 men suspected of being IS members, and 70,000 women and children, many of them related to IS fighters.
What is Turkey proposing to do?
Mr Erdogan told the UN General Assembly on 24 September that he intended to eliminate the “PKK-YPG terrorist structure” and establish a “peace corridor”, where he proposed building 10 “districts” and 140 “villages” to house at least 1 million Syrian refugees.
What do Syria’s Kurds say?
“Turkey’s unprovoked attack on our areas will have a negative impact on our fight against [IS] and the stability and peace we have created in the region in the recent years,” the SDF warned on 7 October. “We are determined to defend our land at all costs.”
An ally of President Trump, Senator Lindsey Graham, called the US move a “disaster in the making”, while the UN’s humanitarian co-ordinator said it was “preparing for the worst”.
Mr Trump later threatened to “totally destroy and obliterate” Turkey’s economy if it took any action he considered “off-limits”.