In reply to Postmanpat:
I guess, and it is only a guess, is that the ultimate priority of the US remains a free democratic world with the US as its major player. Short of that it wants a free democratic US and in the Middle East at least stability and preferably US dominated stability.
A lot of the answers above imply that, given the complexities of the region, and given the undermining of US credibilty (abroad and domestically) resulting from the Iraq debacle, pretty much any of the above outcomes were and remain impossible.
Personally I think that a third Bush administration would have pressed on the the Middle East (pure supposition of course) but realistically the very best outcome they could have hoped for was to topple Assad but end up simply with an Iraqi + crisis, a still very ,unstable region and a huge terrorist backlash in the West. And that's if the Russians and/or Iranians
didn't fight back.
The other option for Obama would have been to do just enough to dissuade the Russians from getting involved, but without boots on the ground, which could have resulted in being dragged into an endless civil war, unclear who the US were allied with or who they were fightingnagainst.
So maybe Obama took the least bad option?