Like many of you, I also hope we don't get involved, but frankly I don't think we have much of a hand to play militarily anyway. Putin knows this, Obama knows this... I think we all know this.
IMO... our moral authority is pretty eroded given our various adventures since 2001. Longer if you want to go back through other fiascos.
It *could* turn into a bit of a proxy war within the EU, but my hunch is nobody will do much unless the US will also come to the party. I find it all unlikely though unless the EU is willing to give up Russia's oil/gas/energy imports.
I do not idealize Putin like many seem to, but he appears to be a good tactician.