Turkey has been threatening to attack the Kurdish-controlled towns of Afrin and Manbij for some time in an attempt to counter a militia called the People's Protection Units (YPG), which it considers a terrorist group.
The YPG denies any direct military or political links with the PKK.
The US also rejects Turkey's portrayal of the militia, which has proven to be a key ally in the battle against the jihadist group Islamic State.
The YPG is part of an alliance with a number of ethnic Arab militias called the Syrian Democratic Forces (SDF). With the help of US air strikes, its fighters have captured tens of thousands of square kilometres from IS.
The US dissuaded Turkey from also attempting to take the mainly Arab town of Manbij from the SDF by force, but got the YPG to agree to withdraw east of the River Euphrates. Turkish officials say that has not yet happened.
One recent development appears to have brought forward Turkey's plans to clear YPG fighters from Afrin and Manbij.
News broke on Saturday that the US was helping the SDF build a new "border security force".
US officials said the plan was to train about 30,000 personnel - half of them SDF fighters - to help prevent infiltration by IS militants across the Turkish and Iraqi borders and parts of the River Euphrates, which effectively divides SDF- and Syrian government-held regions.
The announcement enraged Turkey's government. President Recep Tayyip Erdogan said on Monday that he considered the border force to be a "terror army".
So why would the Turkish assault on Afrin matter?
It could bring two Nato allies into direct conflict and also have a major impact on relations between Turkey and Russia.
Russia is believed to have hundreds of troops in Afrin and effectively controls the enclave's airspace. An offensive without its approval might prove difficult for Turkey and open a major new front in a war that activists say has already cost more than 340,000 lives.
Some Turkish troops have been in Syria for three months after entering northern Idlib province following an agreement with Russia and Iran to try to reduce fighting between pro-Syrian government forces and rebel fighters.
The observation posts which the Turkish army says it has established are close to the dividing line between Arab rebel-held land and Kurdish-controlled Afrin.
Turkey’s National Security Council said earlier on Wednesday Turkey would not allow the formation of a “terrorist army” along its borders.
As the council met, a Reuters reporter witnessed the Turkish army deploying nine tanks to a military base just outside the city of Hatay, near the border with Afrin, to the west of the area where the border force is planned. That followed earlier reports of a military buildup in the area.
On Monday, with relations between the United States and Turkey stretched close to breaking point, Erdogan threatened to “strangle” the planned U.S.-backed force in Syria “before it’s even born”.
https://www.reuters.com/article/us-mide ... F62GV?il=0
The YPG has done more to facilitate the downfall of IS than anyone else and is one of the few moderate forces left in Syria, it would be sad for Turkey to repay them like this.