because NATO is getting closer to our borders, we can't leave it without response
oh wait, that wasn't NATO, was it?
But meddling into internal political processes seems like bread and butter for any would be superpower. Or are Iraq and Afghanistan invasion somehow not interference with political processes?
It probably won't end well if everyone decides to "one-up" each other, though, similar to how WW1 started.
This is one of few times I've seen "invade" used to describe Russia's actions in Crimea. I find it strange that even western media talks about "annexation" when they usually employ exaggerated terminology for all kinds of trivial events.
edit: I'm not saying the invasion was trivial, I'm saying the media response was/is understated compared to non-events they exaggerate
Russia saw that as a provocation and a breach of previous agreements.
What should NATO do, when Poland asks to become a member? Say no, because Russia has the right to meddle in Poland? Say no, because not upsetting Russia is more important than Poland's future? Say no, because defending the western half of Europe is all we should care about?
If Russia doesn't like everyone wanting to join NATO, it's really easy for them to fix: Stop making everyone feel threatened by Russia.
(Now, in the real world, that may not work, because Russia is paranoid, because of a couple of guys named Napoleon and Hitler who, starting from further west than the eastern edge of Poland, got all the way to Moscow - or within 15 miles of it, in Hitler's case. So Russia has some grounds for concern based on history. Nevertheless, NATO has exactly zero interest in invading Russia. My main point, however, stands - it's Russia's fault that NATO is expanding, not NATO's fault.)