Turkish President Recep Tayyip Erdogan announced in a tweet that Turkish offensive into northeast Syria has started.
Several large explosions on Wednesday rocked the northeast Syrian town of Ras al Ain, on the border across from the Turkish town of Ceylanpinar, a CNN Turk reporter said, adding that the sound of planes could he heard above.
The operation could reshape the map of the Syrian conflict once again, dealing a blow to Kurdish-led forces that have battled Islamic State while widening Turkey's territorial control at the border.
It was Turkey's third such incursion since 2016. Motivated largely by the aim of containing Syrian Kurdish power, Turkey already has troops on the ground across an arc of northwestern Syria, the last stronghold of anti-Damascus rebels.
WHAT DOES TURKEY WANT?
Turkey has two main goals in northeast Syria: to drive the Kurdish YPG militia which it deems a security threat away from its border, and to create a space inside Syria where 2 million Syria refugees currently hosted in Turkey can be settled.
It had been pushing the United States to jointly establish a "safe zone" extending 32 km into Syrian territory, but repeatedly warned it could take unilateral military action after accusing Washington of dragging its feet.
HOW WILL THE KURDS BE AFFECTED?
The Kurdish-led Syrian Democratic Forces (SDF) have spent years expanding its control across northern and eastern Syria, helped by the U.S.-led coalition against Daesh.
A rare case of a winner in the Syrian war, the Kurds and their allies have set up their own governing bodies while always insisting their aim is autonomy, not independence.
All of this could unravel with Turkey's military advance.
The SDF-affiliated Syrian Democratic Council has said an attack would trigger a new wave of mass displacement.
For the SDF alliance, in which the Syrian Kurdish YPG militia is the dominant force, much will depend on whether the United States continues to keep forces in other parts of the northeast and east.
A full U.S. withdrawal would expose the area to the risk of more Turkish advances, a Daesh revival, or attempts by Iranian- and Russian-backed government forces to gain ground.
Confronted by the prospect of U.S. withdrawal last year, the Kurds beat a path to Damascus for talks over allowing the Syrian government and its ally Russia to deploy at the border.
The talks made no progress, but such negotiations could be an option again in the event of a wider U.S. withdrawal.
HOW FAR COULD TURKEY GO?
The northeastern border region, currently controlled by Kurdish-led forces, stretches 480 km from the Euphrates river in the west to the Iraq border to the east.
The immediate focus of Turkey's military plans appears to be around a section of the border between the towns of Ras al Ain and Tel Abyad, which are about 100 km apart. A US
official told Reuters on Monday U.S. forces had withdrawn from observation posts there.
As well as air strikes on Ras al Ain on Wednesday, a witness in Tel Abyad said the sound of explosions rang out there and residents fled.
Although under the control of Kurdish-led forces, that part of the border has historically had a strong Arab presence.
"It's a region where the population is Arab and where Turkey has good ties with the leading groups," said Ozgur Unluhisarcikli of the German Marshall Fund. If the YPG tries to hold territory there "it will lose a lot of blood," he said.
DO RUSSIA AND IRAN BACK TURKEY'S MOVE?
Russia and Iran, the other two major foreign powers in Syria, strongly support President Bashar al-Assad - unlike Turkey and the United States 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 northeast Syria, the Damascus government - backed by Russia - may try to retake control of much of the region not seized by Turkey.
WHAT IS THE WESTERN REACTION TO TURKEY'S PLAN?
There has been no public support from Turkey's Western allies for its plan to settle 2 million Syrians - more than half of the refugees it currently hosts - in northeast Syria.
A senior State Department official described the plan as "probably the craziest idea I've ever heard".
The main Western concerns are that an influx of Sunni Arab Syrians into the largely Kurdish northeast 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 DOES THIS MEAN FOR AL ASSAD?
While the territory in question is already outside Syrian government control, a Turkish incursion could mean the area switching from a non-hostile force - the SDF - to Turkey and rebels that have sought to topple 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 COULD THIS MEAN FOR DAESH?
Chaos could present Daesh with an opportunity to stage a revival and the SDF has been conducting operations against Daesh 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 Daesh prisoners if the situation was destabilised by a Turkish invasion.
The SDF is still holding 5,000 Daesh 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.