My own guess is that Putin will steal what he believes he "needs," but not everything he wants. Thus, he will conquer the Ukrainian seacoast as far as Odessa, and link it up with the eastern rebel provinces (Donetsk and Luhansk). But he will not actually roll through Kyiv, because:

- NATO would react in some way, even if only financially (by cutting off Russian access to the world banking system)
- He would face some level of guerrilla warfare in the capital
- He would then have responsibility for Ukraine's economic problems
- The legitimate Ukrainian government would flee to Lviv, and get considerable international sympathy as well as overwhelming public support.

The sad truth is that if Putin limits himself to the seacoast and eastern provinces, NATO will do little in response.

Note also that Putin will certainly make up some kind of ridiculous excuse, just as Hitler did. Here is Hitler's famous quote:

[link to www.historyplace.com]
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"I shall give a propagandist reason for starting the war. Never mind whether it is plausible or not. The victor will not be asked afterward whether he told the truth or not. In starting and waging a war it is not right that matters but victory."
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Hitler's series of false-flag attacks was called Operation Himmler.