> exclusively-defensive alliance
This is another myth that needs to die. I'll give three examples:
1. NATO bombing of Kosovo;
2. NATO invasion of Libya; and
3. NATO has strategic nuclear weapons.
Even if you ignore those real examples of aggression, from an adversary's perspective, the only difference between NATO being "defensive" and "aggressive" is a policy change or even just stretching the definition of "defensive" to include preemptive military action in other countries.
Only the mind of one already set on conquering his neighbors could see an invitation to join an exclusively-defensive alliance as an existential threat.
At this point, it's obvious to any country that borders Russia that it's either join NATO or be absorbed in one way or another. If Ukraine falls, it's on to Moldova or Georgia. He already has his pretext provinces ready to go.