From Moon of Alabama:

Tillerson tried to calm the Turks by claiming that the “border protection force” was not for border protection…

“Having the Syrian State in control of its Northern Border wasn’t the only Russian demand. The other was that the Kurds hand back the oil fields in Deir al Zor. The Kurds refused suggesting that the US won’t allow that anyway.”..

In a meeting Russia proposed to the Afrin Administration that if Afrin is ruled by the Syrian regime, Turkey won’t attack it. #Afrin Canton Administration refused this proposal…

Turkey had supported al-Qaeda. Fighting it directly is against Turkish interests. It is itself too vulnerable as many al-Qaeda fighters have family and friends living within Turkey…

Turkish planes are taking off from the U.S. build Incerlik air base to bomb the Kurds in the Syrian west while U.S. tanker planes take off from Incirlik to support the U.S. alliance with the Kurds in the east.

One of the largest US-backed rebel groups announces defection to Turkey

With the loss of Jaysh Al-Thuwar, the Syrian Democratic Forces no longer have a large Arab rebel group in the northern countryside of Aleppo to aid them in their battle against the Turkish forces.

From the comments at MoA:

The Afrin Valley is well-suited for an armored forces dash to Afrin itself, and Afrin is open to self-propelled artillery bombardment from high points to the East…

France requests UN Security Council meeting on Turkish operation in Afrin…

What I think is, who could go and openly confront officially US-backed forces in Syria without risking a world war? Not Iran, not Russia, and not Syria who would have been accused of “bombing its own population”.

Which left only Turkey. Whom the US has to handle with kid gloves, because, you know, Incirlik (to quote the former US Colonel Patrick Lang, “If the Turks close Incirlik to the US, we’re out of business in the ME”) and NATO…

Iran and Syria come out against the Turkish initiative…