NATO, after a huge degree of internal discussion and political wrangling, offered Russian particpation in the NATO BMD system, which is optomised to protect Europe from weapons launched from the Middle East. Clearly, being able to use sensors placed in Southern Russia would increase SA and warning times, and would be similarly beneficial for Russia. This offer was made at the Detroit Summit three years ago in an effort to demonstrate this wasn't an anti-Russian system, but the invitation to participate was stiffly ignored by Russia. Similarly NATO declared - and adopted the equivalent of an 'Open Skies' regime to demonstrate to Russia that there were/are now no NATO plans about defence against a Russian attack.
Sources?
None of this justifies the annexation of Crimea in the way that was executed.
Agreed, but I think it's wise to try and see the different sides of things.