Turkish President Recep Tayyip Erdoğan has long threatened to send troops into northeastern Syria to clear the border region of Syrian Kurdish fighters whom Turkey considers a serious security threat.
A Turkish invasion looked more likely after President Donald Trump's sudden announcement that U.S. troops, who had fought alongside the Kurds against Islamic State group, would withdraw from the area. Both Mr. Erdoğan and a spokesman for Kurdish fighters confirmed a Turkish offensive had begun Wednesday morning, with reports of warplane and mortar attacks.
Here is a look at what Turkey wants to achieve in the area, and the risks and challenges it faces by getting even more deeply involved in the Syrian crisis.
What does Turkey want?
Turkey wants to create what it calls a "safe zone" in a stretch of territory along its southern border with Syria that is currently controlled by Syrian Kurdish fighters, known as the People's Protection Units, or YPG.
Turkey considers the YPG as terrorists affiliated with the outlawed Kurdistan Workers' Party, or PKK, which has waged a 35-year-long battle against the Turkish state. Ankara also views the YPG-controlled zone as an "existential threat."
The YPG denies that it is a terror organization but says it aspires to the ideals of the PKK's founding leader, Abdullah Öcalan.
Mr. Erdoğan has demanded a "safe zone" that is 20 miles deep and stretches more than 300 miles toward the Iraqi border. He initially had hoped to do it in collaboration with the United States but grew frustrated with what he considered to be delaying tactics by the U.S.
Once secured, Turkey wants to resettle the area with 2 million Syrian refugees who fled to Turkey due to the conflict in their home country. How such a massive resettlement would be carried out is unclear. Human rights groups have warned that any escalation of fighting in the area could displace hundreds of thousands more people.
Mr. Erdoğan has spoken of plans to build towns, villages, hospitals, and schools but also says Turkey, which has already spent some $40 billion on the refugees, cannot afford to do it alone. He has said he will convene a donors conference to help meet the cost and has called on European nations to share the burden, warning that Turkey could be forced to open the "gates" for an influx of migrants to Western nations.
Kurds vow to fight back
Turkey has carried out two previous incursions into northern Syria in recent years with the help of Syrian rebels. In the first offensive in 2016, Turkey pushed back Islamic State group militants west of the Euphrates River. In the second operation last year, Turkey captured the Syrian-Kurdish controlled enclave of Afrin. Those regions are currently administered by Turkish-backed opposition groups who run them as virtual Turkish-administered towns.
Analysts say this operation would likely be more complicated. Unwilling to let go of an area they wrested from the Islamic State group, the battle-hardened Kurdish fighters – trained and equipped by the U.S. – have vowed to fight the Turks until the end.
"It's a huge area for the Turkish military to go into and clearly there will be resistance on the part of the [Syrian Kurdish forces]," said Bulent Aliriza, of the director of the Turkey Project at the Washington-based Center for Strategic and International Studies.
Mr. Aliriza suggested the operation may be a limited one that does not stretch all the way to the Iraqi border. "That's what we are going to look at first. How deep and how broad is it, whether it's all the way across from the Iraqi border to the Euphrates, or just limited to two or three penetration points."
Critics of Mr. Trump's decision fear a Turkish operation could have destabilizing consequences for the region, while both Democrats and Republicans have warned that a Turkish attack could lead to a large number of fatalities among the Kurds, who are holding thousands of captured IS fighters and their families.
What about the fight against the Islamic State group?
One of the big question marks surrounding Turkey's plans is whether fighting the Syrian Kurdish forces would allow IS to make a comeback.
Turkey insists that the global battle against the militants won't suffer, and points to its 2016 incursion, which drove away IS from another border region.
But Kurdish officials have warned that they would have to divert their forces away from guarding IS prisoners in case of a Turkish assault. Kurdish authorities run more than two dozen detention facilities, scattered around northeastern Syria, holding about 10,000 IS fighters.
The White House has said Turkey will take over responsibility for the imprisoned fighters, but it is unclear how that would happen, if it all.
Mr. Erdoğan says Turkey and the United States are working separately on plans to repatriate foreign fighters held in Kurdish prisons.
This story was reported by The Associated Press.