The two largest militaries in NATO are issuing threats, warnings and ultimatums... to each other
turkey us war

Safe to say Russia and Syria won’t be crying themselves to sleep if this escalates
Turkey demands US military pulls out of northern Syria to the west of Euphrates around Manbij. US refuses to do so. Turkey threatens it will launch an offensive on Manbij whether US is still present there or not. Turkey adds it may not be able to distinguish between Kurdish militiamen and US troops in battle and could target both. US sends a hi-ranking Pentagon delegation to Manbij to deliver the message the US "will respond aggressively if attacked". - This has all happened. Welcome to 2018 where the nations with two largest militaries in NATO are issuing threats, warnings and ultimatums...to each other.

Pentagon's loyal New York Times makes sure its message is relayed:
Two senior American generals came to the front line outside the Syrian city of Manbij on Wednesday flying outsized American flags on their vehicles, in case pro-Turkish forces just the other side of the no man's land, 20 yards away, did not realize who they were.

"We're very proud of our positions here, and we want to make sure everybody knows it," said Maj. Gen. Jamie Jarrard, the Special Operations commander for the American-led coalition in Iraq and Syria.

If the message to Turkey was not clear already, the overall coalition commander accompanying General Jarrard, Lt. Gen. Paul Funk [three stars], elaborated. "You hit us, we will respond aggressively. We will defend ourselves."

The trip was the first by such senior United States military officers to the front in northern Syria since Turkey's president threatened to attack the city of Manbij, calling it a bastion of terrorists and demanding that American forces leave.

But the Americans have refused, creating the potential for an unprecedented armed conflict between two NATO allies, the United States and Turkey - the latest twist on the seven-year-old war in Syria.
But whether US and Turkey do come to blows or not, it is already true that Turkish proxies are taking pot shots at US troops, and US-trained proxies are being decimated by its own NATO ally:
American policymakers worry that the Afrin conflict, and the threat against Manbij, will degrade their Kurdish and Arab allies.

"I think our main concern is that anything that disrupts everybody's focus on ISIS and eliminating the complete physical caliphate - and we're close, we're very close - something people couldn't have imagined a year ago - anything that disrupts us or takes our eye off that prize, is not good," General Jarrard said.
To make everything weirder this is happening while the two militaries continue to cooperate elsewhere. US is still using Turkey's Incirlik airbase, and the US may be secretly supplying Turkey's offensive on Afrin with AWACS data.
US soldiers

US soldiers where they shouldn't be