The problem is that in Donetsk and Lugansk there are up to million ethnic Russians.
Another problem is that NATO acts aggressively by putting low range missiles in neighboring to the Russian Federation states. Which means less than five minutes to strike Moscow.
Of course, that the Russian Government sees this as a threat to its national security and what they can do now is to enter Donetsk, Lugansk, and divide the former Ukraine into three brand new states. The first most Eastern situated will directly join the Russian Federation. The central state will be Russia-friendly. And the new Western state will be more like Poland 2, but with high probability to lose lands to Romania, Slovakia and Poland.
The Russians have missiles capable of destroying easily the US air carriers, which technically renders them as useless, expensive and luxury deadhouses.
And this is before the Chinese enter to help its military allies...