US will not get involved militarily. Sanctions will be it.
Remember how Russia took The Crimea from Ukraine in 2014? Same thing happening here again. Russia says the people of these areas want to be part of Russia again so they are just helping them escape from the Ukrainian government that will not allow them to make their own decisions. But a lot of this is because Russia (or at least former KGB officer Putin) believes the Ukraine is Russian territory and is obviously slowly trying to annex it all. It remains to be seen if Latvia, Lithuania, Estonia, Moldova, and Belarus will suddenly really, really want to be part of Russia again or not.
TLDR : Putin trying to get a lot of the Iron Curtain Gang back together by hook or crook. No other countries are going to enter the fighting.