The Donald may pretend to be tough (I guess it's from his WWE background), but when he comes up against someone who is not playing a game, Donald backs down, just like the bully he his.
In this case it is Turkish President Recep Tayyip Erdogan that forced Donald to cower.

Just two weeks ago, this was the news: U.S.-Backed Force Could Cement a Kurdish Enclave in Syria

Then just days later, the Trump Administration changed its tune.

Tillerson told reporters “that entire situation has been mis-portrayed, mis-described, some people misspoke. We are not creating a border security force at all.”

Except Tillerson was lying and everyone knew it.
Erdogan, being an authoritarian bully himself, sensed weakness and pressed the issue.
He attacked the Syrian Kurds and increased the warmongering rhetoric.

US-Turkey tensions mounting surrounding the Turkish invasion of the Syrian Kurdish Afrin District look to be getting much worse, with Turkey’s deputy premier Bekir Bozdag warning that the US is the one risking confrontation by supporting the Kurds.
Bozdag warned the US needs to “review its soldiers and elements giving support to terrorists on the ground,” and that US forces who continue to support the Kurdish YPG, which Turkey considers terrorists “will become a target in this battle.”
The US isn’t believed to have any serious forces inside Afrin District, which Turkey is presently invading. Turkey’s President Erdogan, however, says they will attack Manbij next, and there are a substantial number of US troops deployed there.

When has one NATO country ever threatened war against another NATO country? Ever?

So today Donald backed down.

Turkey said Saturday that Washington has pledged to stop giving arms to YPG Kurdish forces in Syria, as Turkey's offensive against the U.S.-backed group there enters its eight day.
Turkey's presidency said in a statement that U. S. National Security Adviser H.R. McMaster spoke Friday with Ibrahim Kalin, a spokesman for Turkish President Recep Tayyip Erdogan. McMaster confirmed in the phone conversation that the U.S. would not give weapons to the YPG militia, the statement said.

On one hand, I'm glad Trump didn't push the issue. It's a war we don't need.
On the other hand, it's a shame that we are abandoning the Kurds yet again. Russia is also abandoning the Kurds. It's the only act of collusion with Russia that Trump has committed.

Lost in all this high-stakes politics is the fact that the Turkish invasion of the Afrin canton has gone nowhere. The heavily outnumbered, out-gunned Kurds have so far fought them to a standstill.
That's in isolated Afrin. What happens if Turkey attacks Manbij?

The FSA and Turkish forces are unable to advances over the past 2 days in Efrin due to the poor weather & have lost some ground to the YPG. Everything is covered in mud. An FSA fighter inside told me they are staying put & waiting for the weather to improve.

— Elizabeth Tsurkov (@Elizrael) January 26, 2018