I'm not sure it isn't an act of war to place unwanted and uninvited military forces in another nation's territory whether you believe they are adequately preventing terrorist activity or not.
Yeah. Like Russian military in Crimea and Eastern Ukraine, and Georgia, and many more places.
The US policy of toppling Assad is as you say "unbelievably stupid". If Trump ends it, more power to him.
The US policy in Syria is not and has never been to topple Assad. It is and was to destroy ISIS.
Believe me, if the US wanted to, it could topple Assad within a month or two. Especially now that the Syrian Arab Army (SAA) has been all but destroyed and consists mostly of only militias from Iran and Hezbollah and such. And of course the Russian Air Force.
If we leave Syria "soon" as Trump proposes, he will be making the same mistake that Trump himself blamed Obama for leaving Iraq too soon (with Al Qaida in Iraq not destroyed, it turned into ISIS).
As sidd mentioned : once you are there, you need to own it and take responsibility.
And as Neven said : "Send in the military, enforce a ceasefire, then start the rebuilding process of 10-20 years."
If the cooler heads prevail, we are going to be in Syria for a long time. As we should, to rebuild that country.