I think it's fairly straight forward. Ideally capture or if necessary destroy Ukraine as to prevent Ukraine turning into, from the Russian perspective, an 'anti-Russian Russia'. The vision is that both Belarus and Ukraine are extensions of the Russian state and as such a Western aligned Ukraine isn't going to be tolerated.
My knowledge of Russian bureaucrats comes mostly from watching Chernobyl on HBO, but I wouldn't be surprised if it isn't simply that Putin said he wanted Ukraine and nobody at the top could push back.
My knowledge of (not Russian but) post-Soviet bureaucrats comes from personal experience, I speak fluent Russian and follow their news/politics sporadically but over the past few decades. I think the way they are portrayed in Chernobyl is brilliant. It's a structure so permeated by apathy, corruption, bullying and grandstanding that it simply cannot effectively transmit information in either direction, whether top-down or bottom-up (but especially bottom up). It's clear that Putin was fed a steady diet of his own wishful thinking / propaganda, and he bought into it fully. If he had a realistic view of the world around him, he would have prepared a lot better for this war of aggression that he staked everything on.
And, as an aside, I think COVID started for the exact same reason as what's portrayed in Chernobyl. Chinese model of governance is not that different. When accurate sober assessments of facts on the ground are needed and speed is of the essence, its ineffectiveness takes over. Once the genie is out of the bottle and there's nothing to hide from superiors, and once Beijing sends over some adults to whip things into shape, the sheer scale of China's economy allows it to power through and suppress the crisis using draconian means. But for the first who knows how many weeks (certainly well before Jan 1 2020), it was much slower motion Chernobyl.
There certainly are reasons (not a value judgement), some of them cynical:
- Crimea wants to control the canal from the Djnepr, which had been shut off by Ukraine.
- Russia wants the coast line to control more gas fields in the Black Sea. The U.S. of course also want to control them by proxy (see Burisma/Biden). Snake island also controls some of the fields.
- Rightly or wrongly, Russia is scared of NATO expansion.
I think it may be the case that the great Russian empire theory is a pretext that is put forward by Putin for his own population and eagerly taken up by the West for propaganda. Attacks on Kyiv (stalled) and Kharkiv could be diversions from the south. As noted above, the Kharkiv attack does not make sense though.
The reasons for the immediate action might be logical, but looking at the medium term consequences it breaks down completely in my mind. The only two possibilities I can see going forward is either a forever-war as in Afghanistan, or a forever-sanction situation like Iran or DPRK.
I don't see how any listed benefits would outweigh the costs of either option. It troubles me a lot, because Putin has seemed to me to be a carefully calculating player up until this. It makes me wonder if he has truly lost it. Or if there is some grand plan which this is only a small part of. Both are scary.