It is about NATO expansion to Russian border. Putin probably want eastern ukraine under his control to serve as buffer. Ukraine really made a terrible mistake for signing up with the europeans disregarding its half ethnic russian population. The only way for Ukraine to get out of this mess is if Ukraine agree to keep EU and NATO at distance, which I doubt it will agree to considering Russian army now amass at the border. I'm not sure whether Russia will invade or not, but it is a good time to watch US resolve to help its proxy.

As for China, I think it is time to reconsider China's military project with Ukraine. Ukraine might be forced to suspend it under US pressure.