My understanding is those 2 areas have had some form of continuous conflict between Ukrainian forces and the Russian separatists. Now that Putin says Russia is recognizing their independence, it opens the door for his invasion. He has ordered Russian troops into those areas as “peacekeepers“. But to the West, once Russian forces cross the border, they’re invading Ukraine because it’s Ukrainian territory. And since those areas have been in continuous conflict, Russian troops being there will make it easy to say they were targeted by the Ukrainian forces and therefore justify Putin’s military actions against Ukraine.
This is how I understand how this can all play out. Oh and don’t think China isn’t watching all of this considering their desires to invade Taiwan.