It really is tiring listening to the Russian apologetics coming from other posters in this thread.
"Russia felt threatened"
No. Nobody wants to invade Russia.
The deal since the end of the cold war has been this: Russia can do whatever the hell it likes inside its own borders. Its oligarchs with Putin at the front can rob and pillage the country to their hearts content. Putin can use his loyal FSB to suppress the opposition, rig elections, and dominate the domestic media and brainwash his population to believe whatever he want them to believe. The rest of the world will do no more about this than hand out tiny wrist slaps, while holding their noses and trading with Russia. They kept their seat in the UN security council. They were invited into G7. They were treated as an equal to larger, richer, liberal, democracies.
The only reason there is war in Ukraine is because this incredibly generous deal was not enough for Putin and his ilk, they wanted more. More influence, more power, more vassals. They want to restore the mythical glory of the Russian empire, or the Soviet empire, or both. They want all the territories that Russia has ever held dominion over. Getting control over Ukraine would have given that dream a big territory, population, and economical boost.