Kaur, it's difficult to see the strategic view when one is under impression of recent events and situations.

Look at a map, and remember how central to Russia it is to have a buffer in front of Moscow. NATO would be within 500 km of Moscow if Ukraine joined. He has to prevent this at almost any cost, or else his successful poker game about the Crimea would enter history books as totally backfired and disastrous to Russia strategically.

Russia can also not gulp the Ukraine in its entirety. We know how they got Chechnya under control; they flooded the country with more troops than there were civilians. They cannot do this with the Ukraine, even the Chinese couldn't pull this off.
There's no substantial Russian population in the Ukrainian territory the most close to Moscow, though.

https://en.wikipedia.org/wiki/Demogr...ne#Nationality

So even if Putin was able to bite off all Ukrainian areas with a relative majority of Russians he would merely guarantee that the remainder would seek an alliance with the West - and NATO would at Moscow's doorsteps (by Russian standards).

The threat of inviting the Ukraine is a huge and reusable bargaining chip of the West.