You mean NATO with its orders of magnitude power encroaching East despite their empty promises in 90s?
That definitely came out of nowhere and not because Russian soldiers occupied Crimea and effectively occupied east of Ukraine. Ukraine gave up nuclear weapons and Russia was one of the guarantors of Ukraine’s sovereignty.
And regardless of all of this, Ukraine is a sovereign nation and can do whatever the f””” it wants.
Ukraine signed Budapest memorandum in 1994 and willingly gave up nukes for security assurances (ironically Russia is one of the guarantees).
So there is no choice for Russia: fight or be dismembered.