Over the course of history, Russia has been invaded 50 times, so they're pretty touchy about that regardless of how peaceful things look at the moment. With the

Over the course of history, Russia has been invaded 50 times, so they're pretty touchy about that regardless of how peaceful things look at the moment. With their current borders, they have no real geographic barriers. The Soviet Union had pretty good barriers (mountains and such), needing to guard just nine passes, which can be done with relatively limited troops. They currently control just one of those passes. Taking Ukraine restores their control over two more, and provides a base to go further. Meanwhile, Russia has horrendous demographic problems. Their fertility rate dropped to something like 1.2 children per woman when the Iron Curtain fell, and hasn't recovered. So now their military-age population is aging out. They're not going to have the population to defend Russia with the borders they have now, and right about now is when they start losing the ability to invade countries like Ukraine to expand their borders.