I heard a very interesting input on this in an interview on CNN or MSNBC yesterday which made some sense to me (regretfully I don't have a source for it at the moment, but I think I remembered who said it, so I will try to find a link on the net and post it later*).
According to the person interviewed he/she (I think it was a she) said that the reason for Russias action against Ukraine is not primarily because of any NATO expansion. Instead it is because Ukraine is a functional democracy which is/would be threatening to Russia which is an authoritarian state. If Ukraine is a functional, prosperous democracy which is looking to the West, people in Russia may start to realize that their lives could become better if Russia was a functional democracy. This reasoning makes quite much sense to me.
Nevertheless, Putins recent speech about historical grievances in general (with dubious historical accuracy) was worrying to hear.
* Update: I've made an initial search but could not find anything. If I remember correctly the reasoning came from a female author of history books (Ann