The honest answer is no one knows. The line is subject to change. My opinion is foreign troops threatening Russia.
Hypothetical (I hope for now and forever) If, say, France actually put soldiers in Ukraine Russia would do all sorts of shenanigans short of nuking anyone. If French and Ukrainian troops entered Russia, then we’d see tactical nukes used on Ukrainian soil for certain.
I just respect everyone and never assume their identity is what I initially (subconsciously?) think. It took a long time to get there, but I just take every person as a person worthy of respect and dignity as my default.