The US has confined itself to supply-and-support assistance for rebels in Syria so far - though the rationale for supporting the rebels at all (when judged against US interests) is not obvious.
To actually intervene, and probably settle things, the US needs some sort of game-changer such as use of chemical weapons by the Assad regime.
And what do you know......! "Evidence" of just this very thing is beginning to emerge. Invasion ahoy (after a suitable "WMD style" soften up), get the popcorn ready...
US - Syria used chemical weapons on a small scale - RTÉ News