NATO is expanding and Russians were not happy about that way before current situation. Being Crimean myself I would say that so much feared 'annexation' was forced on to Russia by Crimean local establishment. Not that Russia was against it though. As a matter of fact - throughout 23 of the Ukrainian independence it was never less than 65% of Crimeans that were ready to join Russia.

Ukrainian crisis is the result of building nationalistic state in the country with two different cultures, frequently incompatible. And also grave incompetence of the Ukrainian government leading to escalation of the conflict.

Poland and Baltic states - these countries are outright hostile toward Russia, yes, it has historical reasons. But now it's easier to trade and use economic power than to conquer and control territory. These countries are part of EU, and Russia will never - until forced - engage in any military confrontation with them.