The "conquest" of Crimea was a response to an implied threat to a natural gas trade with Europe in the followup of a western-sponsored coup.

(to be even more specic, Crimean events were mostly about regaining the control of the Black Sea Fleet naval base, while Donbass events were about reacting to a hostile coup, but the two things are closely related)

It was also an object lesson to all the minor powers in the region: you cannot take US money to have a small victorious colour revolution anymore. Not in Russia's backyard.

edit: oh wow downvotes, I am so surprised /s

Please don't go on about downvotes. It's against the site guidelines, is tedious, and usually turns out to be wrong. In the end your post got almost no downvotes relative to upvotes.

https://news.ycombinator.com/newsguidelines.html

I think it was the approach that a former ally (Ukraine) would open up the door to NATO/USA and Russia didn't like having a USA naval base right on its doorstep, and losing control of the Black Sea. There may already be USA'S military bases nearby (Romania, Bulgaria, etc) but not THAT close.