There are several ways the US can act as a spoiler, as indeed they have been doing over the last 9 years, without physical presence in Syria, the token forces are enough. Turkey as we saw can heavily mobilise and send vast troops across land borders and be supported by nearby NATO countries either from East Europe or even UK and France if needed.
I think the battle is now shifting to an economic warfare. As the price of crude plummets, US will suffer a major blow both in self sufficiency and in maintaining the high production cost fracking for oil. The economic consequences of Corona virus on the world economy will presumably hit western economies more than it will Russia. As always I think Putin is playing the long game and capitulation may come unexpectedly from unforeseen directions.