>>509529531
It's their public red line.
From their perspective it's fight for it or lose it anyway, which do you think they'd choose?
The whole reason Russia is fighting this war in Ukraine is because of NATO encroachment, they want a buffer zone between them and Russia because without it Russia is effectively conquered.

So long before they lose the buffer zone they'd probably turn their entire western bordered countries into radioactive zones which will turn them into buffer zones that way.