Turkey v Syria's Kurds: The short, medium and long story
The Turkish military is carrying out an operation in north-eastern Syria against a Kurdish-led militia alliance previously allied to the United States.
We've boiled down why it matters.
Turkey considers the biggest militia in the Kurdish-led alliance a terrorist organisation. It says it is an extension of a Kurdish rebel group fighting in Turkey.
Turkey's president wants a 32km (20-mile) deep "safe zone" along the Syrian side of the border clear of Kurdish fighters. There, he wants to resettle up to two million Syrian refugees living in Turkey.
The Turkish offensive began after US troops, who relied on the Kurds to defeat the Islamic State group, withdrew from the border area.
The Kurdish-led alliance has now sought help from Syria's government, which views the US as an enemy.
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 terrorists, and 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 multinational coalition.
Despite the risk of a confrontation with a Nato ally, Turkey carried out cross-border operations in 2016 and 2018 to contain the YPG.
When the US declared the military defeat of IS in Syria in March, Turkish President Recep Tayyip Erdogan pushed President Donald Trump to create a "safe zone" in north-eastern Syria.
To avert an offensive, the US agreed to establish one together with Turkey, but called it a "security mechanism". The YPG complied and began dismantling border fortifications.
Two months later, US troops pull back from the border after Mr Erdogan told Mr Trump that Turkey was about to begin an operation to set up a "safe zone" alone.
The SDF said it had been "stabbed in the back" by the US and that an offensive would reverse the defeat of IS.
On 9 October, Turkish troops and allied Syrian rebels launched a ground assault on SDF-held territory. Mr Erdogan said they aimed to "neutralize terror threats against Turkey" and facilitate the return of Syrian refugees.
Four days later, with the Turkish-led forces making gains and the death toll mounting, the US announced a full withdrawal from northern Syria. The SDF turned to the Syrian government for help and reached a deal for the Syrian army to deploy along the border and counter the Turkish offensive.
Why is Turkey worried about Syria's Kurds?
It feels threatened by the People's Protection Units (YPG), the military wing of the 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 say they are separate entities.
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 against the jihadist group Islamic State (IS), SDF fighters captured tens of thousands of square kilometres of territory in north-eastern Syria between 2015 and 2019.
The SDF set up an autonomous administration to govern the region, home to more than two million people. It avoided conflict with the Syrian government, but called for recognition of Kurdish autonomy.
Is this Turkey's first cross-border operation?
Despite being part of the US-led coalition against IS, Turkey opposed the support the US gave the SDF.
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.
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 a major operation to expel YPG fighters from Afrin. Almost 300 civilians were reportedly killed in the fighting.
What prompted the talk of a "safe zone"?
In December 2018, President Donald Trump declared that IS had been defeated and announced that the the 2,000 US troops helping the SDF in Syria would 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 suggested creating a "20-mile safe zone".
Although the US withdrawal was delayed, Turkish President Recep Erdogan declared that 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 agreed to set up a joint "security mechanism" on the Syrian side of the border to address Turkish security concerns. They avoided using the term "safe zone" but said the area would be clear of YPG fighters. US and Turkish troops carried out joint patrols in the area and the YPG began dismantling border fortifications.
But on 6 October, Mr Erdogan told Mr Trump that a cross-border operation would "soon be moving forward", according to the White House. Mr Trump responded by saying US troops based in the area would withdraw and not support the operation.
The White House also said Turkey would be responsible for all captured IS fighters in the area. The SDF is detaining 12,000 male suspects in makeshift prisons, while 70,000 women and children suspected of links to IS are being held at camps.
What is the aim of Turkey's offensive?
At the start of "Operation Peace Spring" on 9 October, Mr Erdogan said he wanted "to prevent the creation of a terror corridor across our southern border, and to bring peace to the area".
"[We] will neutralize terror threats against Turkey and lead to the establishment of a safe zone, facilitating the return of Syrian refugees to their homes," he added. "We will preserve Syria's territorial integrity and liberate local communities from terrorists."
SDF commanders warned that a Turkish offensive would "spill the blood of thousands of innocent civilians" and might pave the way for the re-emergence of IS.
Mr Trump threatened to "totally destroy and obliterate" Turkey's economy if it took any action he considered "off-limits". A senior US official said that included ethnic cleansing or firing at civilians.
What is the situation on the ground now?
Over the first five days of the assault, Turkish-led ground forces targeted a sparsely populated, mostly Arab area between the towns of Tal Abyad and Ras al-Ain. However, densely-populated, mainly Kurdish areas to the west and east also came under bombardment.
Dozens of civilians were reportedly killed and 150,000 others displaced. Meanwhile, IS suspects' families detained at camps in Ain Issa escaped after the area was shelled.
On 13 October, Mr Trump decided to withdraw all US troops from northern Syria.
Later, the SDF reached an agreement with the Syrian government for the Syrian army to enter its territory and deploy along the border.
The army moved into SDF-held areas the next day, setting up a potential clash with the Turkish-led forces.