The Turkish military has launched a major cross-border operation in north-eastern Syria against a Kurdish-led militia alliance allied to the United States.
The move came after US troops, who relied on the militia alliance to defeat the Islamic State (IS) group on the ground in Syria, withdrew from the border area.
We've boiled down why it matters.
$Why has Turkey launched an assault?$
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.
Turkey's president wants a 32km (20-mile) deep safe zone along the Syrian side of the border clear of Kurdish fighters. He also hopes to resettle up to 2 million Syrian refugees currently living in Turkey there.
The Kurdish-led alliance says it will defend its territory. It accused the US of 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 carried out two cross-border offensives against the Kurdish forces.
In early 2018, it attacked the Kurdish enclave of Afrin, in western Syria. Dozens of civilians were killed.
That December, with IS close to defeat, President Donald Trump said the US would begin withdrawing troops from Syria. When commanders and allies expressed concern about the fate of the Kurds, he proposed creating 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.
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In August, the US and Turkey agreed to establish one together, but called it a security mechanism. The YPG complied and began dismantling border fortifications.
But two months later, Mr Erdogan warned Mr Trump that Turkey was going to set up a safe zone alone.
Mr Trump said US troops would withdraw from the border area and that Turkey would be responsible for all IS militants detained by the SDF.
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.
The SDF said it had been stabbed in the back by the US, and warned that a Turkish offensive would 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).
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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.[1]