A couple weeks ago I pointed out that the situation at Manbij was a powerkeg. It's only gotten more volatile since then.

Last weekend, the Kurdish YPG attempted to preempt an attempted Turkish invasion of Manbij by handing over a ring of villages around the outskirts of the city to the Syrian military, who positioned troops there. The gamble was that Turkey wouldn’t directly attack the Syrian military to try to get at the Kurds.
Which appears to have been untrue, as Syrian military officials reported heavy shelling from Turkish military forces nearby, killing at least eight Syrian troops and wounded 10 others across several villages in the area.

Now technically, if the Syrian troops fire back and kill Turkish troops (which will eventually happen unless something changes), the United States would then be obligated under the NATO treaty to side with the Turkish forces.
But that isn't how the American forces are playing this.

Dozens of Army troops, riding Stryker armored vehicles waving American flags, drove outside the Syrian town of Manbij in a mission aimed at keeping U.S. allies Turkey and Syrian Kurds from fighting each other and focused instead on the fight against IS.

Which begs a number of questions: what if this bluff doesn't work?

But the Americans are stepping into a crowded space in northern Syria, where U.S.-backed Kurdish groups, Turkish and Russian troops, Syrian government forces and Islamic State militants are all within firing range of one another.

Other questions include: what exactly is our mission in Manbij? What if our troops get killed by Turkish troops or their jihadist allies? What if our Kurdish allies ask us for our help?
And most importantly, what about the fact that our troops are stationed nearby Syrian and Russian troops, and that ISIS is no longer in the area?

By last weekend, Russia brokered a deal with the YPG and Syria, where Syiia would ring Manbij with defensive forces, Russia embedded troops with Syria, and the US embedded troops in Manbij with the YPG. All this and Turkey set a serious powderkeg, and with Turkey already shelling Syrian troops, it’s almost certain to explode.

The situation is seriously FUBAR.