> NATO membership is entirely voluntary.

Yes and Russia doesn't deny that either. The request was for NATO to stop allowing countries like Ukraine to volunteer. Is that unfair ? Yes, it is a bully move. But that was the bully's request, and the West's stance is, effectively, we don't negotiate with terrorists. It's the attitude of: whatever, we do whatever we want. That's also fair, but don't be surprised why Russia proceeded, because they can have the same sentiment: whatever, we do whatever we want. The magnitude of recklessness is outsized, but the direction is the same.