StevieTNZ said:
What threat does Ukraine pose to Russia, if Ukraine join NATO?
Only Putin can answer with certainty. All else would be speculation.
However, from my perspective, Putin is reacting to prior comments and/or tacit commitments, not to expand NATO to the east toward the Russian border, and especially, not Georgia or Ukraine. He's demanded so at different times. Apparently, he feels his demands have been ignored, NATO has expanded, and it makes him look powerless, or less powerful.
Certainly, NATO is defensive. But that is not good enough in some minds.
More recently, two things I've heard about Zelenskyy, but have not independently verified:
1. Apparently someone in the government made a statement recently, in the past several weeks or months(?), that Ukraine would not join NATO. Zelenskyy apparently contradicted that statement indicating his desire for Ukraine to join NATO sooner than later. That would complicate the matter of Donbas and Luhansk provinces, and Crimea.
2. Apparently, very recently, may be in the past few weeks or last month, Zelenskyy talked about acquiring nuclear technology (for nuclear energy, see one of my previous posts), and apparently there was a comment about nuclear weapons. I seem to remember some comment, but I don't recall when or the context. That would certainly attract Putin's attention and raise his hackles.
There have been some comments about the Budapest Memorandum, but I don't know what was said and who said what.
https://en.wikipedia.org/wiki/Budapest_Memorandum_on_Security_Assurances
In the years that followed [break up of USSR], Ukraine made the decision to denuclearize completely. In exchange, it would get a security guarantee from the U.S., the U.K. and Russia, known as the Budapest Memorandum.
I know about the denuclearization of CIS and a joint program by US and Russia, and I know some folks who worked on it.
Besides the fact that Ukraine is much less pro-Russian that it was three decades ago. One of my friends believes we'll see an attempt to split Ukraine into a Pro-Russian East and pro-West/EU West, but I can't see that being palatable. The politics are complicated.