I suspect NATO announcing it was considering having Ukraine as member was a major tuning point for Russia's worries about it's security related to this country- which was likely a major factor contributing to the "chain of" current events. Having a "western-military-allied" nation on your doorstep surely must have concerned Russia. Was it not a similar situation that what got the USAs attention n the 60s in Cuba? Remember the Cuba blockaid standoff and"the Bay of Pigs" incident.