The US State Department urged Turkey to be "scrupulous and avoid civilian casualties" while ensuring that its military operations remain limited in scope and duration.
"There's no stepping back from Afrin", he said in a speech in Ankara. But now the US government has effectively told the Syrian Kurds that they can keep the huge chunk of Syria they control for the indefinite future.
So from Turkey's perspective, they've already waited out ISIS, and see it as time for their long-anticipated attack on the Kurds.
Additionally, as was reported by Middle East Eye earlier today, January 20th, 2017, "Around 25,000 Free Syrian Army rebels are joining the Turkish military operation in northern Syria with the goal of recapturing Arab towns and villages seized by the YPG Kurdish militia nearly two years ago". But the Russian intervention in 2015 turned the tide in President Bashar al-Assad's favour.
Without elaborating, he said that Turkey reached an agreement with Russian Federation - whose forces back Syrian President Bashar Assad - over the operation.
What's particularly interesting/concerning about this story is that up until July of 2017, the Free Syrian Army had been directly funded/armed by Barack Obama and the United States Government dating back to 2013.
The Kurds "will surely put up hard resistance, but I don't know how heavily armed they are and a determined Turkish attack is probably difficult to fend off", he said.
The militia has been a key ally of the U.S.in its fight against IS, with Washington providing air cover as well as weaponry during the YPG's fight to recapture IS strongholds in Syria a year ago.
Iran, Mr Assad's other main military supporter, called for a halt to the operation.
Washington's relationship with Moscow has been sour for some time, amid disagreements involving the war in Syria, the conflict in Ukraine and the Kremlin's alleged meddling in the 2016 U.S. presidential elections, among others.
It is true that in earlier stages of the Syrian civil war the regime was fighting less against IS than against other Syria opposition groups, mostly as a reflection of geography and of who posed an immediate threat to the more heavily populated regime-controlled areas in the western half of the country.
But Ankara has denied inflicting civilian casualties, with Foreign Minister Mevlut Cavusoglu accusing the YPG of sending out "nonsense propaganda and baseless lies".
Mattis said Ankara gave the US military advance notice of its Afrin offensive.
US officials say 's furious response - and a possible military strike - have only escalated the crisis.
He said Kurdish forces had recaptured a key hill in the region, briefly seized by pro-Turkish Syrian rebels on Monday.
Who sanctioned this commitment to a new army, which, if Syria and its Russian and Iranian allies, and the Turks, do not all back down, risks a major US war with no allies but the Kurds? Both the Obama and Trump administrations have provided the group with training and weapons as the chief USA ally against ISIS in Syria.
According to Max Hoffman, associate director and a Turkey expert at Center for American Progress, Turkey is "deeply unhappy" with the existing balance of power in northern Syria -- where the regime and the Kurds are ascendant.