Russia doesn't even need to invade Ukraine. Eastern Ukraine can just declare that it will hold referendum to join Russia or establish a new country. I suspect that is what going to happen soon. There is not much Ukraine can do if the referendum happen, except try to invade eastern Ukraine. Russian army built up is probably to prepare for the referendum, to protect eastern Ukraine from invasion. It is interesting to see what the west will do if referendum happen. I suspect it will be like what happened in Crimea, all barks and sanctions. The world can see the hypocrisy as usual.