It's difficult for me to imagine a negotiated peace that doesn't involve NATO guarantees of sovereignty for the remaining parts of Ukraine. Otherwise, what's the value from Ukraine's perspective?
So one way or another this will end in short order. Or it will be WW3 (not necessarily nuclear, but it's a risk).
Governments and organisations do tend to violate agreements if and when it suits them. Look at NATO expansion...
"After speaking with many of those involved and examining previously classified British and German documents in detail, SPIEGEL has concluded that there was no doubt that the West did everything it could to give the Soviets the impression that NATO membership was out of the question for countries like Poland, Hungary or Czechoslovakia."[0]
So one way or another this will end in short order. Or it will be WW3 (not necessarily nuclear, but it's a risk).