Ukraine striking back inside of Russia makes perfect sense to me. And normally I wouldn't differentiate or comment as long as the targets were military targets.  I just expect them to take out the forces that are attacking them -- the weapons depots, air bases, launchers, that kind of thing.  Even if it was civilian infrastructure that was critical to the military operation like roads or bridges it would make sense to me as a normal part of war.

And radar normally counts as fair targets too.

But taking out Russia's early warning nuclear defense radar.... that's where my spider senses start tingling.  Are those the military assets that threaten Ukraine the most?  I don't think so.  Those are the ones that protect Russia and that maintain the nuclear stalemate that is keeping all of us alive. This bodes far worse than anything I've ever heard before.