Putin, and his great tribe of nostalgic Russians, which by the news footage seems to be comprised middle-aged men in bluejeans and flags, and old women fondly cradling pictures of Stalin. The common denomintor? Grievance at the embarrassment and hardship that followed the collapse of USSR. Putin, with all his shirtless photos with tigers and guns and Siberia taps into this sense of inadequacy and shows Russians that yes, they can still be macho and powerful. It's simplistic primal bullshit, an evolutionary holdover that served proto-humans well when we had simpler brains and scarce resources, but it still works.
Putin didn't have to annex Crimea the same way Bush/Blair didn't have to invade Iraq and Afghanistan and any number of past and present "leaders" great and small who, if not for circumstance and ego, would be pissed-up football hooligans smashing each other senseless. There's no good reason why Russia or the USA, or the north bit of Korea couldn't have sensible governments with decent relations with their neighbours and the rest of the world. We have too many weapons, and the world is now too small for these kinds of spoilers.
We as a species need to lose this ancient holdover that splits us into my team and your team, and have us following madmen (and sometimes women) with guns on whatever fateful path they start down.
If we don't, the archaeologists of whatever species evolves to replaces us will be digging up our fossils and pondering what happened.