The United States has more or less tried to push through its influence right into Russia's door step. That is itself cause enough to put up strong defence.
Which is only really important when the states in question border Russia directly, in which case the disparities in relative power are already massive. Georgia may even have been on the high end of the spectrum.
Leaving out, of course, China and the Ukraine. A direct confrontation with NATO is out of the question.
The question now becomes whether Russia will prefer a stronger military or a more robust economy, and the current administration seems to have decided that the latter is more important. They use the military, prudently, to score political victories.
As for International law, there has been precedent in which a state was oppressed sufficiently to grant it statehood. The case in point, would be E. Timor, via referendum, no less.
There's also been precedent against it -- in Darfur, for example, or in Tibet.
At the end of the day, Russia acted not out of principle, but out of interest, as all actors do. And even then, it wasn't to uphold an idea, but to answer a Georgian misstep (even stupidity).
The ultimate point is that Russia's leaders think they have bigger fish to fry, and I'm inclined to agree.
Russia is still a nuclear power, right? As long as they have that umbrella, they can modernize as slowly as they'd like without major interference from other world powers. That's leaving aside the fact that they're a major energy exporter, so anyone who tried to seriously mess with them would be frowned upon by those who rely on Russian energy, like Europe.
I agree with your point about the nuke umbrella.
I disagree that Russia's energy bludgeon is as effective as everyone insists. For a short-term tool, it's very powerful. But, eventually, the Russians will have to eat their own energy -- they're not going to refine, use, or sell all of what they aren't selling to Europe, and it's not going to be easy to pipe that to some other buyer, even the Chinese. In the mid- and long-terms, it's Russia that feels the crunch and the disadvantage with that approach.
Let's hope he buys some more of those Sukhois after seeing our ships, since every little money he pays to Rosoboronexport will help to make the Su-35BM and other new machinery 
Would be cool to see these guys up close.
below OHP. 