Russia has always (somewhat fairly) been vocal that since the collapse of the Warsaw pact, Nato has no reason to exist other than to defend against a potential attack from Russia.

Nato have always acted as though that's true, and have pursued expanding the alliance to countries bordering Russia, and established missile defence installations there.

The closest we ever got to nuclear war was when Russia moved missiles to Cuba, so it doesn't take a geopolitical scientist to understand that the US is very uncomfortable about threats near its borders, but expects Russia to accept it the other way round.

I'm not up to date with what the propaganda systems are belching out in Russia, but I believe the original reason was something about Nazis in Ukraine, and it would not surprise me if the narrative is shifting to be 'Nato is creeping up on us again'. Which is odd, because Russia already has borders with Nato countries, and controlling Ukraine actually brings them closer to where Nato already has a strong foothold.

A lot of what's going on in Ukraine at the moment is linked to Sevastopol, I think. Russia absolutely cannot afford to lose Sevastopol. Not sure quite what happened that triggered the full scale invasion after the annexation of the Crimea, mind.

Edit: added a quite important “not”