My impression is that Russia (Putin) feels threatened by NATO. That may not be the threat of invasion. It might be just the threat of being no longer the dominant power in the neighborhood, and therefore unable to push people around.
But I think Russia may in fact fear invasion. Germany went from "no threat" to "building up their military but having a treaty with Russia" to "invading Russia" in nine years. If that invasion had been launched from the eastern Ukraine instead of from the middle of Poland, it very plausibly could have broken Russia.
Russia doesn't have half of Poland as a buffer anymore. It doesn't even have (most of) Ukraine. I think that Russia is extremely over-sensitive - even paranoid - given the current circumstances. But circumstances change, sometimes rather rapidly. Given history (not just Germany, but also France), Russia has some grounds for fearing Europe.
"But Europe is different now!" True... until it's not.
Look, I'm not defending Putin. But he's not totally nuts to be worried.