War in Ukraine?

I read through some threads, some wiki articles, and this thread. But I still don't understand why this is such a 'hot' issue for the U.S. to be concerned about. Even considering Russia's involvement in this issue, what advantages would the Ukraine give Russia if they decided to annex the country via military proxy that would lead Russia towards being more competitive in terms of economic and military strength? As far as I know, there are more flaws than draws if Russia pushes ahead in terms of the long term benefits since it will give the U.S. an excuse to launch sanctions against Russia and cripple them economically. That leads to the U.S. benefiting from a hostile Russia move towards the country since it will likely loosen the country from Putin's influence and allow the U.S. to gamble on another president that would be more open towards cooperation with their foreign policy.

Russia does not want Ukraine to go towards the EU because it knows if that happens, it will be very likely that Ukraine will join NATO as well. Russia is still very on-edge about NATO in general and will not tolerate having a NATO member that close. Also Russia is a raw export economy still and Ukraine is very dependent on Russia for its energy.

Also there's a good article about China's foreign policy dilemma with Ukraine in this week's Economist.