I have a theory. Hear me out.
While every nation has some degree of national paranoia, Russia, due to its vulnerable geography (no real natural shields) for the first few centuries of its existence and frequent invasions from foreign forces as a result, experiences what I'm going to call radical ontological paranoia. This is to say that Russia is not only constantly concerned about its own identity in relation to others (caught between East/West) but is concerned about the very viability of the Russian state itself due to the Russian Core having no natural protection zones. The result was a multi-century project of expansionism on a scale the world has never seen. Russia thinks it has to be expansionist in order to survive. Go get the Caucasus, go get the deserts and mountains of Central Asia, go get the Carpathians, plug the gaps, and try the best as possible to plug the northern European plain.
Obviously this still leaves a profound mark on the Russian psyche, even today, especially since it no longer controls some of these buffers I just referred to.
Okay, here's my theory: if Russia requires expansion for its very survival, why doesn't it just expand and take over the entire world? Make the entire globe "Russia," and then the Russian Core will have nothing to worry about anymore. A buffer "state" in that of the Moon, and a zero-gravity, oxygen-less environment like outer space is one hell of a natural barrier between those who would still choose to invade.
What say you?