East ukraine is already lost.Any further attempt by means of arms and extreme actions ,pogroms ,and you will have russian army in East Ukraine. I don't expect them entering central or western ukraine though.There is lack of support there for Russia.
And why do you think that the USA would consider that a bad thing, in a cynical way? I mean Russian army in E Ukraine. Their biggest problem now is EU not having a coherent policy towards Russia, plus Obama being undecided. An invasion would solve all that.
Think about it: E Ukraine isn't Crimea, it can't be quarantined be holding two roads. It also doesn't have the same Russian majority as Crimea - meaning many will fight against the Russian troops, adding to the numbers coming from W Ukraine. We're talking Cecenia, only 10 times worst.
Add to this NATO weapons and training for the W Ukrainians coming through, the galvanization of the EU countries around USA, the immediate sanctions that would obliterate the Russian economy and probably new independence movements in the Caucasus.
But ultimately, it's all about the Ukrainians, and if they decide to fight or not...and the sad part is that it's not in EU's or USA's interest to decimate Russia..