I think in the postmortem we will figure out that this was about #2 moreso than anything else. He wanted NATO off his doorstep but NATO kept encroaching since he did nothing every time they encroached. This is somewhat the act of a madman, but it's a response to NATO continually breaking promises.
The land bridge to Crimea is nice, too, don't get me wrong.
> it's a response to NATO continually breaking promises
I hear this a lot, but when I look into it all I can find is Putin claiming that the US Secretary of State assured Gorbachev in 1990 that “NATO will not move one inch further east.” Gorbachev himself has stated that this quotation is taken out of context, and refers specifically to the movement of military structures within East Germany.
Did NATO put any pledges in writing, not to expand into Eastern Europe? It would be odd if they did, because an open door policy is a founding principle of NATO.
This is somewhat the act of a madman, but it's a response to NATO continually breaking promises.
It's not, actually. The history around this is widely misunderstood.
See also: https://news.ycombinator.com/item?id=42187155