National Geographic had a great documentary on Syria. It explained the history: how Assad's father, a very cruel man, became the dictator and his second-born son, Bashar, became his successor.
Also showed how Bashar had 2 critical turning points in his governance - after 9/11/2001 he could have sided with the U.S. and the West but did not; instead, he opened his prisons and let members of ISIS loose to fight against the allies. And ten years later in the 2011 Arab spring, he could have acceded to his people and given them some semblance of democracy; instead, he used chemical weapons to brutally repress them. The Arab spring in Syria almost succeeded - according to the documentary, the rebels were within a few miles of Assad's compound when Russia, at the last minute, stepped in to side with Assad. Since then the U.S. and Russia have been fighting a proxy war in Syria.
There is no doubt that if/when the U.S. withdraws there will be a power vacuum and many more Syrians will die. Also no doubt that Russia will be happy to have a foothold in the area and access to its oil and other resources.
So is Trump withdrawing to make Russia happy? Or is it really because we've realized, yet again, that we can win battles but there will be no end to the occupation. I don't know what the answer is either except that as Americans, we should acknowledge our part in this and accept more Syrian refugees who have been caught in the middle.
As someone else already said, if we leave are we willing to watch this happen? or were we only there in the first place because of oil?