We're not leaving. Trump wants to destroy ISIS and you can't do that by withdrawing.
The absolute best thing right now would be to team up with them and Assad, clean out the rebel areas (not using our own troops) and get Syria back to some semblance of normality. And then pull out. Politically, however, that won't happen.
Therefore the only thing to do is to force the issue and see if they're bluffing.
The Russians have to know that despite their short supply lines and what counts as interior lines for them that they cannot win a protracted battle with US forces.
I mean maybe if we invaded them they could but they can't match our overseas force projection despite living in the neighborhood.
And the very second they cause US deaths the public won't let the war stop until the Russians are pushed back to their own borders. No ships allowed out of port, planes that venture out of Russian airspace shot down and so forth.
I don't know what the end point would be but there would be very few happy days for Vlad & Co. and they have to know that. Therefore they're bluffing (95% chance).