The problem here is that in Syria we have the murderous Assad regimen on one side and ISIS and other radical Sunni Islamic groups on the other.  The Assad government represents a minority Alawite (Shia) population, and a loose coalition of secular Syrians and other religious minorities, backed up by Iran and Russia.  The opposition is a bunch of non-cooperating splinter groups including ISIS, Kurds and various other Sunni groups.  If we side with Assad (as Trump was inclined to do until this week), we are backing Iran and turning a blind eye towards the Assad governments murderous and illegal use of WMDs on his own people.  If we back the opposition, it's hard to separate ISIS from the other Sunni groups, and backing the Kurds alienates NATO ally Turkey.  In the end, Obama came to the conclusion that the best course was to try and gently force Assad out and leave his government in charge.  Trump will probably eventually come to the same conclusion and policy.