A large part of the responsibility for the crisis was the total ineptness of the EU leadership which blithely pulled Ukraine into the EU regardless of the inevitability that this would ignite tensions within the country – getting Yanukovych to sign an association agreement and then when he was deposed in a coup after he changed tack, then rushing ahead and getting the new regime to sign the agreement before even elections had been held. The EU did this, apparently unaware that joining the EU was by no means a universally widespread wish in Ukraine – in Eastern regions the figure was 19% and in the South 26%!  In fact of course – they cannot have been unaware. They just don’t count people who disagree with them. (These are the regions Russia has now attempted to annex). This was shockingly irresponsible.
Now of course Ukraine is finding out that the promise of a European future was not all it seemed. Even if Ukraine survives the war intact it will be a long time before the country meets the standards to join. 
As Mearsheimer says they have been led down the “Primrose Path” by the EU. The EU fanned the hopes of a pro-EU faction even though they could hardly deliver on these hopes anytime soon and, in the process, set a light to a tinderbox of internal tensions.
And this is just the EU angle. The US dragging Ukraine into NATO was a provocation to Russia. Again, it seems that it was more the idea of Ukraine in NATO that appealed to the US. Jeffrey Sachs reports that in private conversations with US officials he was told there was no imminent prospect of Ukraine joining NATO.
It seems all these people leapt at the notion of pulling Ukraine into their sphere of influence without taking into account the reactions that would set off, internally in Ukraine and externally, with Russia. (This June 2014 Guardian article is perhaps rather revealing. The liberal consensus appears to have been Russia would be “resigned” to this state of affairs and would focus on the Eurasian Union).
- https://www.bbc.com/news/world-europe-61383632 “Ukraine bid to join EU will take decades says Macron – BBC”