The "why" is a complicated question that goes to the core character of the nations involved. Russia by nature feels insecure about her borders, and always has.

The "why" is a complicated question that goes to the core character of the nations involved. Russia by nature feels insecure about her borders, and always has. There's a long history of invasion that has left a trauma to this day. The one thing that Russia fears more than anything else is encirclement. To this end they have over the centuries embarked on numerous territorial expansions and treaties to create buffer zones between themselves and potential enemies, the most recent of which was the USSR (which proved too costly to maintain). When the Russians couldn't keep their "union" going, they entered into negotiations with Western powers about what would happen with these now-free states. This is where things get a little murky, because the Russians insist that they'd received assurances that NATO would not expand into former Soviet states, whereas the West emphatically maintains that no such assurances were given. The higher ups in the Kremlin firmly believe that the West broke its word, and is thus untrustworthy. As NATO expanded further and further Eastward, Russia's insecurities intensified, and the Ukraine was considered to be the last bastion of resistance to encirclement. This is why Russia has spent the past decade meddling in Ukrainian affairs, and why they've stepped up meddling in American politics (Trump is an isolationist and nationalist, which is why Russia favored him). As to what triggered this latest move, that's anyone's guess. Maybe they got some intel that the Ukraine really was going to gain full NATO membership. Maybe not, and they just got a little too worried about it... We'll probably never know. But either way, this current invasion is a drastic effort to keep the Ukraine out of any military alliances with the West.