But if that is his motivation, it's a very risky one. It presumes (wrongly, I think) that all of Eastern Ukraine would willingly embrace the idea of becoming part of Russia. That could lead to a civil war. That could mean non-trivial disruptions to gas supply lines from Russia to Europe and that would be very costly to Russia.

Truth is the always the first casualty of war and I'm not claiming to "know" anything. But recent US history is replete with examples of terrible consequences for blindly picking a side quickly and overlooking that side's shortcomings. The current Mexican Stand Off is not a good thing for anyone, imo. Too many have died already. Both sides falling back and agreeing to sit down at a table and have talks seems to me to be a much better solution than escalating rhetoric, sanctions and armaments.