What can the United States do against ISIS in Syria -- and could it work?

If the United States is serious about thoroughly defeating ISIS, it must -- somehow, some way -- go through Syria.
But how? And in what way?
Those are the big questions now, as President Barack Obama weighs what to do inside the war-ravaged nation where ISIS leaders are based and where the Islamist terror group rose to prominence.
Obama ceded Thursday that "we don't have a strategy yet" for what to do about ISIS inside Syria, with a senior administration official adding that a deci