Because it has long been argued in the West by different people that Russia should be invaded or nuked. John von Neuman, one of the smartest guys who ever lived was crazy enough to argue that the US should launch a first-strike on the USSR with nuclear weapons as soon as possible. Patton, Churchill and others entertained the idea of continuing the war by invading Russia. Around 1917 a western force did in fact invade Russia to fight communists. It is not very well known in the histroy books as it was a small force and it failed. Still, I doubt Russia has forgotten.
People love to talk about appeasment as the reason why Hitler was not stopped, but the uncomfortable truth is that many Western powers had decided that they thought it would be a good thing if Hitler invaded Stalin. That is why they stalled so long. Stalin pleaded for a long time with the West to join him against Nazi Germany. I doubt the Russians have forgotten that either.
In the West we don't remember these stories, because every nation likes to present the rose tinted view of their own history. But seen from Russian eyes the West is a bunch of two-faced liars ready to throw Russia under the bus whenever the opportunity presents itself.
You call NATO a defense organization, yet NATO has never actually defended against any invasion force. Quite the contrary every NATO engagement has been aggressive, attacking other countries. Sure, you can call some of this warranted such as bombing Serbia to stop genocide, but you cannot call it pure defense. It was clearly an offensive action.
If you compare the armed forces of Nordic countries, there is a clear difference between Norway, which is a NATO member, and Finland which is not. Finland has primarily weapon systems for defense. In Norway we have had to shift a lot of our military spending towards offensive capability becuase that is primarily what NATO does, attak other places. We joined NATO bombing Libya, thinking we helped civilians. Yet it turns out we got sucked into a regime change plot hatched by the US, UK and France. It is a bit of a joke to call this "defense."
NATO is all too often used as an offensive force to advance Western interests abroad. You can imagine how this looks from Kremlin.
Please understand I am not siding with Putin or the Kremlin here. I am simply pointing out that we undermine our own moral standing by constantly using NATO in an offensive position. It undermines the narrative that NATO is a defense packt. If you tell somebody in the middle east that NATO is purely for defense, they will laugh in your face.
Finally there is nothing that suggest Putin is planning an invasion of half of Europe. Where is your evidence of that? What is even the rational? It makes absolutely no sense.
