[Matt W]

Putin claims that Ukraine gave Russia no choice. Invasion had to happen in order to protect the Russian state and citizens.

AFAIK, this is almost true. At least in a very cynical way.

Let me explain.

1. Russia is a petro-state. Its only real money-maker is pulling hydrocarbons out of the ground and selling the stuff to Europe.

2. A major Russian pipeline to Europe goes through Ukraine and the Ukrainians used to charge a large fee to allow this transit. So, the Russians stopped using it and have built new pipelines (and are trying to build even more). However, if Russian troops take control of this pipeline, Putin gets a much simpler/cheaper solution than building a NEW pipeline (Nordstream 2 will be eye-wateringly expensive)

3. MASSIVE oil and gas deposits have recently been discovered off the coast of Ukraine. If these were to be exploited... there would be no need for Europe to buy Russian oil. Russia would soon be bankrupt. However, Russia now has troops on most of the relevant shorelines...

4. Crimea gets 85% of its fresh water from one specific Ukrainian river. Or rather... it used to. Ukraine built a dam in revenge for the Russian takeover of Crimea. Crimea has water rationing - but its economy is being destroyed. However, Russian troops have now reached that dam...

5. Before the invasion, there was only one land-based link between Russia and Crimea. And that's a single bridge that could be easily destroyed. Of course, this is assuming that it doesn't fall down without anyone's help. (seismic activity and shoddy Russian construction are not a good combination). Russian troops have now linked Crimea with Mother Russia.

6. Putin is greedy but he's not stupid. It is quite possible that the Invasion was intended to protect the Russian hydrocarbon industry. So, if we're optimistic, it may be that Putin could be content with:
a) keeping Ukraine out of the oil and gas business,
b) seizing road and rail links to Crimea,
c) re-opening the old pipelines.

Anything else would be "nice to have" but those 3 things would protect the Russian economy and safeguard the vast wealth of Putin and the other kleptocratic Oligarchs. Oh, and the Russian citizens in Crimea (including sailors in one of the Russian Navy's two warmwater ports) wouldn't inconveniently die of thirst.