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'm going to tackle this as best as I can. I am not a subject matter expert, but have done enough political science work and worked with both Power Transition Theory and Great Power Theory to at least kick off a discussion. None of what follows is my personal opinion on the war or ideas concerning morality or just wars. This is also very simplified.
-At the moment, the Ukraine War is contained. It is not spreading and, thus, the world powers are not interested in intervening. Even in this case, the amoral state (read Richelieu) has no reason to get involved.
-The war's continuation, at the moment, does not threaten state survival to anyone outside of Russia and Ukraine. Maybe Belarus? But I view that as a non-issue since they are essentially Russia's puppet state.
-Internal challenges in nation's that could intervene will prevent them from doing so. Why? Escalating to "boots on the ground" has one of two effects. One, a surge of nationalism that allows the state to absorb immediate shocks and unifies the population. Two, a complete disruption of legitimacy and systems that could cause the state to collapse. There's not enough risk to justify the possibility of two happening.
-The western European states have not seen a major ground war in Europe since WW2. Entire generations have no idea what a modern nation-state vs. nation-state war is actually like. Afghanistan or Iraq, where international forces did operate, was very different. Getting into a shooting war directly with another power is a huge risk and huge unknown.
I'm not reading the article but instead trying to be amusing. If it breaks the reality, please put me in a new one with really good scotch, healthy knees, and a spirit of adventure!
My family invested in a jungle-gym when I was a kid. We were lucky. The slide was wood with a thin wax coating. It lasted about one year in that region, baking in one season, swelling in another, freezing and thawing in the other two, until it became a splinter distributor and we never used it again.
For the metal slides, however, lying on a skateboard + metal slide = somehow never broke a bone.
I highly recommend you visit your local library and request/check-out a copy of the book Polarization by Nolan McCarty. Read that.
Thanks for this. Time to play around and experiment.